[ARM] Improve proc_setup_smp_id() for unknown CPUs

Currently, the proc_setup_smp_id() function would just hang when
running on unknow CPUs.

This not fare because with multiplatform generic board available
for all ARM families, people will try Xvisor binary on unknown
boards which are never tried before.

This patch improves proc_setup_smp_id() so that for unknown CPUs
we consider Affinity0 field of MPIDR as CPU ID of unknown CPU.

Signed-off-by: Anup Patel <anup@brainfault.org>