clocksource: arm_arch_timer: Fix a NULL Pointer dereference

In case best_frame is not populated with virtual/physical capable frame,
NULL pointer is passed as an argument to of_iomap .To avoid  NULL pointer
dereference add a check before.

Change-Id: Ia7bad6630662ee3a4ad7c9226f9d3858b2e7a36c
Signed-off-by: Ankit Mishra <ankimi@codeaurora.org>