clocksource/drivers/arm_arch_timer: Hack: Expose interface to shared timer

To synchronize SCP and kernel timestamps, a shared memory-mapped
timer is used as a reference. Deltas between:
- SCP timer and the shared timer
- kernel timer and the shared timer
are subtracted to obtain the delta between SCP and kernel timestamps.

TODO: It is not guaranteed that a memory-mapped timer will be
found on all platforms.

Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>