ARM: VGIC: carve out struct vgic_cpu and struct vgic_dist

ARM: VGIC: carve out struct vgic_cpu and struct vgic_dist

Currently we describe the VGIC specific fields in a structure
*embedded* in struct arch_domain and struct arch_vcpu. These members
there are however related to the current VGIC implementation, and will
be substantially different in the future.
To allow coexistence of two implementations, move the definition of these
embedded structures into vgic.h, and just use the opaque type in the arch
specific structures.
This allows easy switching between different implementations later.

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