KVM: ARM: vgic: abstract access to the ELRSR bitmap

Move the GICH_ELRSR access to its own functions, and add them to
the vgic_ops structure.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>