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>

arch/arm/boot/dts/aspeed-g5.dtsi

@@ -1346,6 +1346,11 @@ pinctrl_sda2_default: sda2_default {
 		groups = "SDA2";
 	};
 
+	pinctrl_sgpm_default: sgpm_default {
+		function = "SGPM";
+		groups = "SGPM";
+	};
+
 	pinctrl_sgps1_default: sgps1_default {
 		function = "SGPS1";
 		groups = "SGPS1";