powerpc: gamecube/wii: flipper interrupt controller support

Add support for the interrupt controller included in the "Flipper"
chipset of the Nintendo GameCube video game console.
The same interrupt controller is also present in the "Hollywood" chipset
of the Nintendo Wii.

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>