ARM: local timers: Unmask interrupt before new TVAL is set

Level triggered interrupt is deasserted when a new TVAL is written
only when the interrupt is unmasked. Make sure that the interrupt
is unmasked in CTL register before TVAL is written.

Change-Id: I1bbfe262137fb27c0de68a552b07e285f424b259
Signed-off-by: Sathish Ambley <sambley@codeaurora.org>
(cherry picked from commit 9c642ecd6b71bb068c8a931285dc1dce043b137c)