[ARM] generic_timer: Minor improvement to interrupt delivery latency

It can quite often happen that we wakeup VCPU due someother reason
other than timer expiry. When this happens we should check whether
Phys/Virt timer was active and if so then inject phys/virt irq.

This small improvement reduces virt timer interrupt latency by
10 microsecs (approx.) in basic firmware and improves linux
boot-time for Virt-v7x2 guest on host with 1CPU by 0.01 secs.

Signed-off-by: Anup Patel <anup@brainfault.org>