[MIPS] Add missing prototypes to asm/page.h

This patch fixes the following sparse warnings:

>>>>>>>>>>>>>>>>>>
arch/mips/mm/page.c:284:16: warning: symbol
'build_clear_page' was not declared. Should it be static?

arch/mips/mm/page.c:426:16: warning: symbol 'build_copy_page'
was not declared. Should it be static?
>>>>>>>>>>>>>>>>>>

The fix is to add appropriate prototypes to the header
include/asm-mips/page.h.

Build-tested against Malta defconfig.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@movial.fi>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>