arm64: dts: imx8mq: Align iomuxc node name

Node name should be generic, use "pinctrl" instead of "iomuxc"
for all i.MX8M SoCs.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>