[POWERPC] Fix new interrupt code (MPIC endianness)

All U3/U4 based systems are big-endian, not all express it in their
device trees.

Signed-off-by: Segher Boessenkool <segher@kernel.crashing.org>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>