MIPS: RM7000: Make use of cache_op() instead of inline asm

Small cleanup of the cache code to get rid of inline asm, in preparation
to give tertiary cache support.

Signed-off-by: Ricardo Mendoza <ricmm@gentoo.org>
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/1476/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>