clocksource: arm_arch_timer: Extend arch_timer_kvm_info to get the virtual IRQ

Currently, the firmware table is parsed by the virtual timer code in
order to retrieve the virtual timer interrupt. However, this is already
done by the arch timer driver.

To avoid code duplication, extend arch_timer_kvm_info to get the virtual
IRQ.

Note that the KVM code will be modified in a subsequent patch.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>