ARM: gic: define GICH offsets for VGIC support

The GICH_* constants are defined by the GIC HW spec, and even though
they only be used by KVM to begin with, define them generically in gic.h.

Reviewed-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Christoffer Dall <c.dall@virtualopensystems.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>