sunxi: CPU frequency and voltage scaling support added to -current

[Jared McNeill <jmcneill%invisible.ca@localhost>]

Hi folks --

I've added a few drivers to -current that enables cpu frequency scaling 
support (using defacto standard machdep.cpu.frequency.* controls) as well 
as thermal monitoring support for Allwinner H3/A83T/A64 SoCs.

In addition, the Allwinner thermal sensor driver and generic DT frequency 
scaling driver will work together to ensure that your chip doesn't 
overhead; when the temperature warning trip point is reached, the cpufreq 
driver will automatically throttle the CPU until the temperature returns 
to normal. If for some reason this attempt to cool things down fails, a 
separate shutdown trip point will trigger the driver to immediately 
attempt to poweroff the board.

If everything is working, you should see something like this:

  sunxi# dmesg | grep sunxitherm
  sunxithermal0 at fdt1: Thermal sensor controller
  sunxithermal0: interrupting on GIC irq 63
  sunxithermal0: cpu: alarm 90C hyst 15C shut 105C

  sunxi# sysctl machdep.cpu
  machdep.cpu.frequency.target = 1296
  machdep.cpu.frequency.current = 1296
  machdep.cpu.frequency.available = 1296 1200 1008 816 648

  sunxi# envstat
                       Current  CritMax  WarnMax  WarnMin  CritMin  Unit
  [sunxithermal0]
    CPU temperature:    38.000                                      degC


The upstream Linux dts files are missing these controls for most newer 
SoCs, so you'll need to patch them in manually. There are a few pieces -- 
in the SoC-specific .dtsi we need to tell the cpu@0 node how to control 
the clock. You can see an example of this in 
sys/arch/arm/dts/sun8i-h3.dtsi:

                cpu@0 {
                        clocks = <&ccu CLK_CPUX>;
                        clock-latency = <2000000>;
                };

The board-specific files then need to define a power supply for the CPU 
and a list of operating points (CPU frequency + voltage pairs). Have a 
look at arch/arm/dts/sun8i-h3-nanopi-neo.dts and 
arch/arm/dts/sun8i-h3-orangepi-plus2e.dts for a few examples. Note that 
these are very much board specific! So please don't assume that copying 
one of these examples will work for you.

Cheers,
Jared