MIPS: introduce CPU_R4K_CACHE_TLB

R4K-style CPUs having common code to support their caches and tlb have this
boolean defined by default. Allows us to remove some lines in
arch/mips/mm/Makefile.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/3328/


Signed-off-by: John Crispin <blogic@openwrt.org>