Store MPIDR register in pcpu.

MPIDR represents physical locality of given core and it should be used as
the only viable/robust connection between cpuid (which have zero relation to
cores topology) and external description (for example in FDT). It can be
used for determining which interrupt is associated to given per-CPU PMU
or by scheduler for determining big/little core or cluster topology.

MFC after: 3 weeks