ARM: dts: imx28-apx4devkit: Pass unit address and reg to IOMUX node

Pass unit address and reg to IOMUX node to fix the following build
warning with W=1:

arch/arm/boot/dts/imx28-apx4devkit.dtb: Warning (simple_bus_reg): Node /apb@80000000/apbh@80000000/pinctrl@80018000/mmc2-sck-cfg-apx4 missing or empty reg/ranges property
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx28-apx4devkit.dts

@@ -82,7 +82,8 @@ MX28_PAD_SSP2_SS2__SSP2_D2
 					fsl,pull-up = <MXS_PULL_ENABLE>;
 				};
 
-				mmc2_sck_cfg_apx4: mmc2-sck-cfg-apx4 {
+				mmc2_sck_cfg_apx4: mmc2-sck-cfg-apx4@0 {
+					reg = <0>;
 					fsl,pinmux-ids = <
 						MX28_PAD_SSP0_DATA7__SSP2_SCK
 					>;