MIPS: Fix map_physmem for cached mappings

map_physmem should return a pointer that can be used by the CPU to
access the given memory - on MIPS simply returning the physical address
as it does prior to this patch doesn't achieve that. Instead return a
pointer to the memory within (c)kseg0, which matches up consistently
with the (c)kseg1 pointer that uncached mappings return via ioremap.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>