arm: arm_arch_timer: export function to read physical counter

Since the arch_timer is a system-wide block, other hardware in an SoC
can make use of the counter values. Export a way to read the physical
counter for use by other drivers.

Change-Id: Ib3c9f6fb4d1a077ba32ac13d9ff945d11686b23a
CRs-Fixed: 542881
Signed-off-by: Arif Hussain <arifhussain@codeaurora.org>