ARM: arch timer: Set the TVAL before timer is enabled

Currently we enable the timer and unmask the interrupt
before setting the TVAL.
(in commit 9c642ecd)
This could cause a regression by opening a window
where TVAL can be negative (from a previous timer trigger)
and the interrupt unmasked, which would lead to an
immediate trigger, before TVAL is updated with the new value.
We need to set the TVAL after unmasking the interrupt, but
before enabling the timer.

Change-Id: I4989834f5dd50aa72e4e97f9ab15b82931bfdd1c
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>