MIPS: introduce cpu_coherent_mask

Add a mask of CPUs which are currently known to be operating coherently.
This is setup initially to be all present CPUs, but in a subsequent
patch CPUs in a MIPS Coherent Processing System will be cleared in this
mask as they enter non-coherent idle states. This will be used in order
to determine when a CPU within a CPS system may need to be powered back
up, but may also be used in future to optimise away wakeups for cache
operations or TLB invalidations.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>