watchdog: orion: Use the reference clock on Armada 375 SoC

The 25 MHz reference clock has better stability so its use is prefered over the
core clock. Change the Armada 375 clock initialization to use this reference
clock. To ensure the driver is compatible with an old devicetree, also provide
a fallback path which will silently return to the previous behavior.

While here, add the clock specification to the binding documentation.

Acked-by: Jason Cooper <jason@lakedaemon.net>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Wim Van Sebroeck <wim@iguana.be>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>