dt-bindings: pinctrl: imx8mp: Replace the uint32-array with uint32-matrix

The items of mux_reg/conf_reg/input_reg/mux_val/input_val/pad_setting
should be uint32-matrix instead of uint32-array, fix it and improve the
schema and example.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Link: https://lore.kernel.org/r/1582012300-30260-4-git-send-email-Anson.Huang@nxp.com
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>