MIPS: Hardcode cpu_has_mmips=1 for microMIPS kernels

If we built the kernel targeting the microMIPS ISA then the very fact
that the kernel is running implies that the CPU supports microMIPS. Thus
we can hardcode cpu_has_mmips to 1 allowing the compiler greater scope
for optimisation due to the compile-time constant.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/21022/
Cc: linux-mips@linux-mips.org