[1/2 very MD part] Support Raspberry Pi [0-3] in big-endian mode

[Rin Okuyama <rokuyama.rk%gmail.com@localhost>]

Hi,

I've managed Raspberry Pi [0-3] to work in big-endian mode:

- Zero and 1 in earmv6{,hf}eb
- 2 and 3 in earmv7{,hf}eb
- 3 in aarch64eb

Now, everything except vchiq(4) work without problems.

(1) Data written/read to/from mailbox and dmac should be encoded in
    little-endian.

http://www.netbsd.org/~rin/rpi_eb_md_20201115.patch

(2) DMA buffer for vchiq(4) should also be encoded in little-endian.
    However, this require heavy modification to 3rd party sources.
    At the moment, I disabled vchiq(4) for armeb/aarch64eb.

http://www.netbsd.org/~rin/rpi_eb_vchiq_20201115.patch

Continue to part-2 (which will be CC'ed to tech-toolchain@).

Thanks,
rin