powerpc/xics: Trim #include list

Trim unneeded includes from xics.c.  We don't use signals or gfp
flags, we use only OF functions and don't need prom, and the 8259
is now handled by our caller.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>