ARM: dts: imx50: Remove unused iomuxc-gpr node

The iomuxc-gpr node is not used and causes the following dtc
warning with W=1:

arch/arm/boot/dts/imx50.dtsi:286.28-289.6: Warning (unique_unit_address): /soc/bus@50000000/iomuxc@53fa8000: duplicate unit-address (also used in node /soc/bus@50000000/iomuxc-gpr@53fa8000)

Remove the node to fix the warning.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Abel Vesa <abel.vesa@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>