powerpc/amigaone: Default config for AmigaOne boards

CONFIG_CC_OPTIMIZE_FOR_SIZE is selected, because otherwise the kernel
wouldn't boot. The AmigaOne's U-boot firmware seems to have a problem
loading uImages bigger than 1.8 MB.
Signed-off-by: Gerhard Pircher <gerhard_pircher@gmx.net>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>