arm64: gic-v3: Avoid NULL dereferences

LPI allocation requires that the redistributors are configured first.
It's unlikely that offline cpus have had their redistributors
configured, so filter them out right away. Also, assert on any cpu,
not just the calling cpu, in gicv3_lpi_alloc_tables() when we detect
a unit test failed to follow instructions. Improve the assert with a
hint message while we're at it.

Cc: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Message-Id: <20211011160420.26785-1-drjones@redhat.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>