ARM: mmp: avoid to include head file in mach-mmp

pxa910_set_wake() & mmp2_set_wake() are both declared in head files
of arch/arm/mach-mmp/include/mach directory. If we include these
head files in irq-mmp driver, it blocks the multiplatform build.
So adjust the code.

Signed-off-by: Haojian Zhuang <haojian.zhuang@gmail.com>