pinctrl: aspeed-g4: Add mux configuration for all pins

The patch introducing the g4 pinctrl driver implemented a smattering of
pins to flesh out the implementation of the core and provide bare-bones
support for some OpenPOWER platforms. Now, update the bindings document
to reflect the complete functionality and implement the necessary pin
configuration tables in the driver.

Cc: Timothy Pearson <tpearson@raptorengineering.com>
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>