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>