pinctrl: dt-bindings: Add documentation for Aspeed pin controllers

Outline expectations on the pin controller's relationship with the
System Control Unit (SCU) IP through syscon, and document the compatible
strings for 4th and 5th generation Aspeed SoC pin controllers.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>