arm64: timer: Check the timer interrupt state

We check that the interrupt is pending (or not) at the GIC level, but we
don't check if the timer is asserting it (or not). Let's make sure we don't
run into a strange situation where the two devices' states aren't
synchronized.

Coincidently, the "interrupt signal no longer pending" test fails for
non-emulated timers (i.e, the virtual timer on a non-vhe host) if the
host kernel doesn't have patch 16e604a437c89 ("KVM: arm/arm64: vgic:
Reevaluate level sensitive interrupts on enable").

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>