[MIPS] Put cast inside macro instead of all the callers

Since all the callers of the PHYS_TO_XKPHYS macro call with a constant,
put the cast to LL inside the macro where it really should be rather
than in all the callers.  This makes macros like PHYS_TO_XKSEG_UNCACHED
work without gcc whining.

Signed-off-by: Andrew Sharp <andy.sharp@onstor.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>