If pgprot_writecombine is not #defined, asm-generic/pgtable.h will try
to provide a default implementation by #defining it to pgprot_noncached.
However our implementation is an inline function rather than a #define,
so it was never actually used because of the #define in generic code.

Add "#define pgprot_writecombine pgprot_writecombine" to prevent generic
code from re-defining it.
Signed-off-by: Alex Smith <alex.smith@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/10767/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>