ARM: mvebu: Fix the Armada 370/XP timer compatible strings

The "marvell,armada-370-xp-timer" compatible string, together with
the "marvell,timer-25Mhz" property are deprecated and should be
removed from current DT.

Instead, the timer DT nodes are now required to have an appropriate
compatible string, which should be either "marvell,armada-370-timer"
or "marvell,armada-xp-timer", depending on SoC.

The clock property is now required only for Armada 370 so move it accordingly.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>