ARM: 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. Bring back the functionality and expose it
to drivers so that the RPM code and SMEM code have proper global
timestamps to communicate with other processors.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>