ARM: mvebu: Add the reference 25 MHz fixed-clock to Armada XP

The Armada XP SoC has a reference 25 MHz fixed-clock that is used in
some controllers such as the timer and the watchdog. This commit adds
a DT representation of this clock through a fixed-clock compatible node.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Reviewed-by: Mike Turquette <mturquette@linaro.org>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>