MIPS: smp-cps: function to determine whether CPS SMP is in use

The core power down state for cpuidle will require that the CPS SMP
implementation is in use. This patch provides a mips_cps_smp_in_use
function which determines whether or not the CPS SMP implementation is
currently in use.

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