MIPS: mm: Add prototype for function __update_cache

This commit adds a prototype to fix error at W=1:

arch/mips/mm/cache.c:129:6: error: no previous prototype
for '__update_cache' [-Werror=missing-prototypes]

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>