MIPS: introduce CPU_R4K_FPU

R4K-style CPUs have this boolean defined by default. Allows us
to remove some lines in arch/mips/kernel/Makefile.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/3330/


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