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

Perhaps unsurprisingly, there is a considerable amount of duplicate
code between the MMIO and user accessors for ISPENDR. At the same
time there are some important differences between user and guest
MMIO, like how SGIs can only be made pending from userspace.

Fold user and MMIO accessors into a common helper, 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/20231219065855.1019608-2-oliver.upton@linux.dev