kvmtool: ARM: timers: add "always-on" property to the device tree

The new optional property "always-on" indicates that the timers
are, well, always on when used with KVM.

This allows for substantial performance improvement in the guest
(it switches to NOHZ instead of using a periodic tick per vcpu)
and removes a lot of burden from the host (no need to inject tons
of interrupts with the associated rescheduling overhead).

Old kernels that don't understand this property will simply ignore it.

Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>