pinctrl: aspeed: g5: Add pinconf support

Testing for pinctrl-aspeed-g5 was performed on an AST2500EVB 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>