dt-bindings: soc: imx-iomuxc-gpr: Document the CSI mux

dt-bindings: soc: imx-iomuxc-gpr: Document the CSI mux

On i.MX6Q/6DL the following subnodes exist to describe the CSI port muxing:

- ipu1_csi0_mux
- ipu1_csi1_mux
- ipu2_csi0_mux
- ipu2_csi1_mux

As they were not documented, dt-schema emits warnings like:

  'ipu1_csi0_mux', 'ipu1_csi1_mux' do not match any of the regexes:
  '^pinctrl-[0-9]+$'

Add a top-level patternProperties entry for these CSI mux subnodes
and restrict it to i.MX6Q.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>