dt-bindings: soc: imx: add IOMUXC GPR support

Add binding doc for i.MX IOMUX Controller General Purpose Registers

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>