clocksource: armada-370-xp: Enable timer divider only when needed

The current code sets the timer divider bits always. However, when
the 25 MHz timer is enabled, this is not needed and has no effect.
As this causes some confusion, rework the code so the divider is
set only when needed, i.e. when the 25 MHz timer is not in use.

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