ARM: dts: imx7s: correct iomuxc gpr mux controller cells

Per binding doc reg-mux.yaml, the #mux-control-cells should be 1

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Marco Felsch <m.felsch@pengutronix.de>
Fixes: 94a905a7 ("ARM: dts: imx7s: add multiplexer controls")
Signed-off-by: Shawn Guo <shawnguo@kernel.org>