arm64: arch_timer: Save cntkctl_el1 as a per-cpu variable

As we're about to allow per CPU cntkctl_el1 configuration, we cannot
rely on the register value to be common when performing power
management.

Let's turn saved_cntkctl into a per-cpu variable.

Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>