arm64: dts: imx8mm: correct iomuxc-gpr compatible

The IOMUX Controller General purpose register group are unique almost
per SoC, i.MX8MM is not compatible with i.MX6Q. So correct it.

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