ARM: KVM: arch_timers: Wire the init code and config option

It is now possible to select CONFIG_KVM_ARM_TIMER to enable the
KVM architected timer support.

Reviewed-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <c.dall@virtualopensystems.com>