ARM: OMAP: counter: add locking to read_persistent_clock

read_persistent_clock uses a global variable, use a spinlock to
ensure non-atomic updates to the variable don't overlap and cause
time to move backwards.

Signed-off-by: Colin Cross <ccross@android.com>
Signed-off-by: R Sricharan <r.sricharan@ti.com>
Cc: stable@vger.kernel.org
Signed-off-by: Tony Lindgren <tony@atomide.com>