arm: kirkwood: add pinmux option for the SDIO interface on 88F6282

This commit adds a pinmux option, pmx_sdio, to enable the muxing of
the SDIO interface on the 88F6282 SoC from Marvell.
Signed-off-by: NThomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: NJason Cooper <jason@lakedaemon.net>

arch/arm/boot/dts/kirkwood-6282.dtsi

@@ -30,6 +30,11 @@
 				marvell,pins = "mpp13", "mpp14";
 				marvell,function = "uart1";
 			};
+			pmx_sdio: pmx-sdio {
+				marvell,pins = "mpp12", "mpp13", "mpp14",
+					       "mpp15", "mpp16", "mpp17";
+				marvell,function = "sdio";
+			};
 		};
 
 		i2c@11100 {