MIPS: pgtable.h: Implement the pgprot_writecombine function for MIPS

Previously, the pgprot_writecombine function was simply defined
as pgprot_uncached in include/asm-generic/pgtable.h. This is not
optimal for cores that can actually do write-combine memory writes
so define this function to take into account the core's cache coherency
attribute to achieve such behavior.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/7403/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>