pinctrl: aspeed-g5: Add mux configuration for all pins

The patch introducing the g5 pinctrl driver implemented a smattering of
pins to flesh out the implementation of the core and provide bare-bones
support for some OpenPOWER platforms and the AST2500 evaluation board.
Now, update the bindings document to reflect the complete functionality
and implement the necessary pin configuration tables in the driver.

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