arm64: context: Fix ASID limit in boot messages

Since commit f88f42f8 ("arm64: context: Free up kernel ASIDs if KPTI
is not in use"), the NUM_USER_ASIDS macro doesn't correspond to the
effective number of ASIDs when KPTI is enabled. Get an accurate number
of available ASIDs in an arch_initcall, once we've discovered all CPUs'
capabilities and know if we still need to halve the ASID space for KPTI.

Fixes: f88f42f8

 ("arm64: context: Free up kernel ASIDs if KPTI is not in use")
Reviewed-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Will Deacon <will@kernel.org>