[POWERPC] Fixup use of phys_addr_t in mpic code

The mpic_map() and __mpic_map_mmio() need to use phys_addr_t for the
physical address they are passed.

Signed-off-by: Becky Bruce <becky.bruce@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>