pinctrl: Add pinctrl-aspeed-g5 driver

A small subset of pins and functions are exposed. The selection of pins
and functions is driven by the development of OpenBMC[1] on the
AST2500 SoC, particularly around booting the IBM Witherspoon platform.

[1] https://github.com/openbmc/docs



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