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>