ARM64: arch_timer: resurrect cntpct for MSM users

We have some users of the physical counter reading function, and
they need to work when either the cp15 or mmio timers are present
in any configuration.

Change-Id: If37d73be1a75ef33be1e2276d4fa84740d933891
Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>