clocksource: armada-370-xp: Fix device-tree binding

This commit fixes the DT binding for the Armada 370/XP SoC timer.
The previous "marvell,armada-370-xp-timer" compatible is removed and
two new compatible strings are introduced: "marvell,armada-xp-timer"
and "marvell,armada-370-timer".

The rationale behind this change is that the Armada 370 SoC and the
Armada XP SoC timers are not really compatible:

  * Armada 370 has no 25 MHz fixed timer.

  * Armada XP cannot work properly without such 25 MHz fixed timer
    as doing otherwise leads to using a clocksource whose frequency
    varies when doing cpufreq frequency changes.

This commit also removes the "marvell,timer-25Mhz" property, given
it's now meaningless.

Cc: devicetree@vger.kernel.org
Acked-by: Jason Cooper <jason@lakedaemon.net>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>