pinctrl: dt-bindings: Add documentation for Armada 37xx pin controllers

Document the device tree binding for the pin controllers found on the
Armada 37xx SoCs.

Update the binding documention of the xtal clk which is a subnode of this
syscon node.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>
[Fixed gpios node]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>