powerpc: gamecube/wii: introduce GAMECUBE_COMMON

Add a config option GAMECUBE_COMMON to be used as a dependency for all
options common to the Nintendo GameCube and Wii video game consoles.

Signed-off-by: Albert Herranz <albert_herranz@yahoo.es>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>