clocksource: armada-370-xp: Use BIT()

This is a purely cosmetic commit: we replace hardcoded values that
representing bits by BIT(), which is slightly more readable.

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