KVM: arm64: vgic: Use common accessor for writes to ICPENDR

Fold MMIO and user accessors into a common helper while maintaining the
distinction between the two.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20231219065855.1019608-3-oliver.upton@linux.dev