powerpc: gamecube: bootwrapper bits

Add support for the Nintendo GameCube video game console to the powerpc
bootwrapper.

dtbImage.gamecube is a wrapped image that contains a flat device tree,
an entry point compatible with SDload, and an optional initrd.

Signed-off-by: Albert Herranz <albert_herranz@yahoo.es>
Acked-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>