ARM: dts: aspeed: Add AST2600 pinmux nodes

Add them in their own dtsi and include it in aspeed-g6.dtsi to isolate
the cruft.

Link: https://lore.kernel.org/r/20190911165614.31641-2-joel@jms.id.auSigned-off-by: NAndrew Jeffery <andrew@aj.id.au>
Signed-off-by: NJoel Stanley <joel@jms.id.au>
Signed-off-by: NArnd Bergmann <arnd@arndb.de>

arch/arm/boot/dts/aspeed-g6.dtsi

@@ -258,9 +258,4 @@
 	};
 };
 
-&pinctrl {
-	pinctrl_emmc_default: emmc_default {
-		function = "SD3";
-		groups = "SD3";
-	};
-};
+#include "aspeed-g6-pinctrl.dtsi"