limit number of VCPUs on demand

Currently the ARM GIC checks the number of VCPUs against a fixed
limit, which is GICv2 specific. Don't pretend we know better than the
kernel and let's get rid of that explicit check.
We now fail if the number of requested VCPUs could not be
instantiated instead of limiting the number of VCPUs.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>