arm64: timer: Wait for the GIC to sample timer interrupt state

There is a delay between the timer asserting the interrupt and the GIC
sampling the interrupt state. Let's take that into account when we are
checking if the timer interrupt is pending (or not) at the GIC level.

An interrupt can be pending or active and pending [1,2]. Let's be precise
and check that the interrupt is actually pending, not active and pending.

[1] ARM IHI 0048B.b, section 1.4.1
[2] ARM IHI 0069E, section 1.2.2

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