clockevents/drivers/arm_global_timer: Use writel_relaxed in gt_compare_set

Use the relaxed version to improve performance. we measured time of
4096 rounds of gt_compare_set() spent on Marvell BG2Q:

before the patch: 3690648ns on average
after the patch: 1083023ns on average

improved by 70%!

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>