ARM: KVM: arch_timers: Add timer world switch

Do the necessary save/restore dance for the timers in the world
switch code. In the process, allow the guest to read the physical
counter, which is useful for its own clock_event_device.

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