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: NJason Cooper <jason@lakedaemon.net>
Acked-by: NGregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: NWim Van Sebroeck <wim@iguana.be>
Reviewed-by: NThomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: NThomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: NEzequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: NDaniel Lezcano <daniel.lezcano@linaro.org>