FDT for armv5

[Yuri Honegger <yuri.honegger%icloud.com@localhost>]

Hello,
Over the last few weeks, I've been working on adding device tree support for armv5 boards. It is now working well enough that I can boot into a ramdisk, so I'd like to talk about if NetBSD is interested in merging something like this. 

There are two parts to it:
- the general changes required to boot an armv5 CPU using device trees. This adds a GENERIC_V5 configuration, the MMU initialization code and some ifdefs around platform-specific stuff.
- device tree based drivers for the NXP i.MX23 SOC on the olinuxino board. I've used this board for development because it is an armv5 board already supported by NetBSD. For now, I've only converted the absolute minimum to boot NetBSD (interrupt and timer drivers).

I do have a few questions:

- Is it possible to change device trees under sys/external/gpl2/dts/... or will this cause issues the next time these files are synchronized with the linux kernel? 
The i.MX23 has different UART controllers. Currently, only the debug UART is functional, but the unmodified device tree causes NetBSD to prefer another UART where the drivers are missing.
However, for NetBSD I think it would make more sense to switch over to the debug UART for everything because 1) U-Boot for the olinuxino already prefers the debug UART, 2) The debug UART is in my opinion easier to access on the olinuxino board.

- All files seem to contain a revision comment at the start. Is this something that has to be manually edited or is this done by CVS? What is expected from new contributions
Same questions for __RCSID I see in some files.

- Most SOC-specific drivers are prefixed with the manufacturer/soc name, e.g. sunxitimer for the timer for sunxi SOCs. The existing imx23 code doesn't do this. Is it worth changing this for imx23 drivers? (e.g. icoll => imx23icoll or timrot => imx23timrot)
If we change it, change all or just the ones where I've added device tree support?

- What is the preferred way to submit code? I've seen patches on mailing lists, but the website (https://www.netbsd.org/contrib/#submit-fixes) suggests opening a PR. Looking through PRs, they seem to be more bug-focussed than on new contributions.
For reference, my changes are slightly more than 1000 lines.

In the meantime, my code can be seen here: https://github.com/NetBSD/src/compare/trunk...recursivetree:netbsd-src:armv5-fdt-upstreamable

If someone wants to test it on the i-mx23-olinuxino-mini (There are some more steps required to get the ramdisk going, 
I'll skip them to keep it short. This "guide" will panic because I can't find the init process late in the boot process. If someone with the hardware wants them, I'm happy to type them out):
1. Build the u-boot bootloader using
make mx23_olinuxino_defconfig
make boot.sb
2. Format a microSD-card using a MBR with at least two partitions. The first partition must be of type 0x53, the second a FAT partition.
3. burn the u-boot boot.sb to the first partition (dd if=u-boot.sb of=/path/to/first/partition)
4. Build the NetBSD kernel
./build.sh -u -U -m evbearmv5-el kernel=GENERIC_V5
5. Build the device trees
./build.sh -U -m evbearmv5-el dtb
6. Copy the device tree (src/sys/dtb/arm/obj/imx23-olinuxino.dtb) to the FAT partition
7. Copy the kernel uImage (src/sys/arch/evbarm/compile/obj/GENERIC_V5/netbsd.ub) to the FAT partition 
8. Insert the SD card, apply power, and observe the debug UART header at 115200 baud

- yuri