ARM: dts: 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>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Joel Stanley <joel@jms.id.au>