clocksource: armada-370-xp: Introduce new compatibles

The Armada XP SoC clocksource driver cannot work without the 25 MHz
fixed timer. Therefore it's appropriate to introduce a new compatible
string and use it to set the 25 MHz fixed timer.

The 'marvell,timer-25MHz' property will be marked as deprecated.

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>