Test all MPIDR affinity bits

Currently we only test the Aff0 bits of the MPIDR to determine the
'primary' CPU. In multi-cluster systems, MPIDR.Aff{3,2,1} may not be
zero, and there may by multiple CPUs where MPIDR.Aff0 is zero. In these
systems we might determine that two cpus are the primary CPU.

This patch adds a MPIDR_ID_BITS mask, and uses it in all cases we test
the MPIDR, making this safe for multi-cluster systems. This doesn't
bring full support for multi-cluster systems, however, as they may
require additional hardware to be set up (e.g. CCI).

Signed-off-by: Mark Rutland <mark.rutland@arm.com>