clocksource: armada-370-xp: Use CLOCKSOURCE_OF_DECLARE

This is almost cosmetic: we achieve a bit of consistency with
other clocksource drivers by using the CLOCKSOURCE_OF_DECLARE
macro for the boilerplate code.

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>