ARM: VGIC: Adjust domain_max_vcpus() to be VGIC specific

ARM: VGIC: Adjust domain_max_vcpus() to be VGIC specific

domain_max_vcpus(), which is used by generic Xen code, returns the
maximum number of VCPUs for a domain, which on ARM is mostly limited by
the VGIC model emulated (a (v)GICv2 can only handle 8 CPUs).
Our current implementation lives in arch/arm/domain.c, but reaches into
VGIC internal data structures.
Move the actual functionality into vgic.c, and provide a shim in
domain.h, to keep this VGIC internal.

Signed-off-by: Andre Przywara <andre.przywara@linaro.org>
Reviewed-by: Julien Grall <julien.grall@arm.com>