powerpc/amigaone: Bootwrapper and serial console support for AmigaOne

This adds the bootwrapper for the cuImage target and a compatible property
check for "pnpPNP,501" to the generic serial console support code.
The default link address for the cuImage target is set to 0x800000. This
allows to boot the kernel with AmigaOS4's second level bootloader, which
always loads a uImage at 0x500000.

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