MIPS: Move declaration of bcache ops to cache.c

bcache is not tied to CPU's cache interface. Just move those
declaration to cache.c so it can be avaialble to CPU with all
cache types.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>