arm64: dts: imx8mp: use syscon for iomuxc-gpr

It is preferred to use syscon per bindind doc

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>