powerpc/amigaone: Add platform support for AmigaOne

This commit adds the setup code for booting Linux on AmigaOne G3SE (G3
only), AmigaOne XE and uA1 (G3/G4) desktop computers. These boards were
sold by Eyetech and are based on MAI Logic's Teron boards and its
Articia S northbridge.
The AmigaOne uses U-boot as firmware, which doesn't support a flattened
device tree yet. The northbridge has some design flaws, which makes it
necessary to use non cacheable memory for DMA operations
(CONFIG_NOT_COHERENT_CACHE) and to avoid setting the coherence (M) flag
for memory pages.

Signed-off-by: Gerhard Pircher <gerhard_pircher@gmx.net>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>