clocksource: armada-370-xp: Simplify TIMER_CTRL register access

This commit creates two functions to access the TIMER_CTRL register:
one for global one for the per-cpu. This makes the code much more
readable. In addition, since the TIMER_CTRL register is also used for
watchdog, this is preparation work for future thread-safe improvements.

Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>