dt-bindings: clock: add DT binding for the Xtal clock on Armada 3700

This commit adds the DT binding documentation for the the Xtal clock on
Armada 3700 used in the Marvell Armada 3700 SoCs.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>