arm64: dts: imx8mn: update iomuxc-gpr node name

It is better use syscon for IOMUXC GPR, since it contains various
bits for system control

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>