dt-bindings: pinctrl: add imx7ulp pinctrl binding doc

i.MX 7ULP has three IOMUXC instances: IOMUXC0 for M4 ports,
IOMUXC1 for A7 ports and IOMUXC DDR for DDR interface.

This patch adds the IOMUXC1 support for A7.

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: devicetree@vger.kernel.org
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>