powerpc/gc/wii: Remove get_irq_desc()

Fix the following build failures:

arch/powerpc/platforms/embedded6xx/flipper-pic.c: In function 'flipper_pic_map':
arch/powerpc/platforms/embedded6xx/flipper-pic.c:105: error: implicit declaration of function 'get_irq_desc'

arch/powerpc/platforms/embedded6xx/hlwd-pic.c: In function 'hlwd_pic_map':
arch/powerpc/platforms/embedded6xx/hlwd-pic.c:98: error: implicit declaration of function 'get_irq_desc'

These failures are caused by the changes introduced in commit
"powerpc: Remove get_irq_desc()". The reason these drivers were not
updated is that they weren't merged yet.

Signed-off-by: Albert Herranz <albert_herranz@yahoo.es>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>