pinctrl: aspeed: Add SGPM pinmux

Add SGPM pinmux to ast2500-pinctrl function and group, to prepare for
supporting SGPIO in AST2500 SoC.

Signed-off-by: Hongwei Zhang <hongweiz@ami.com>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>