powerpc: Add AmigaOne platform, a subclass of MPC85xx

[jhibbits]

Summary

The AmigaOne platform, encompassing the X5000 and A1222 at this time, is
based on the mpc85xx platform, but includes some things not listed in
the device tree. Some custom devices, like CPLD, could be added to the
device tree with an overlay, or other means. However, some cannot
easily be done, such as the power button interrupt. There's no clean
way to add a device to just sit on the interrupt line, it would be
platform-specific anyway, so just add it as a new platform. The
directory will also become a location to add AmigaOne platform drivers,
such as the aforementioned CPLD, and its children.

[jhibbits]

Don't use config_intrhook_establish(), the mutex isn't initialized yet.

[jhibbits]

Fix the diff

[bdragon]

Tested on X5000 w/ clang9 kernel.