KVM: selftests: Use MPIDR_HWID_BITMASK from cputype.h

No need for a home-rolled definition, just rely on the common header.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240422200158.2606761-19-oliver.upton@linux.dev


Signed-off-by: Marc Zyngier <maz@kernel.org>