pinctrl: aspeed: g4: Add pinconf support

Testing for pinctrl-aspeed-g4 was performed on an OpenPOWER Palmetto
system, using the strategy outlined in the commit message for the
change to the Aspeed pinctrl core.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>