imx8mm-venice-gw700x: prepare to synchronise device trees with linux

As a preparatory step remove the pinctrl_pmic reference which does not
exist in the Linux upistream device tree.

This avoids the following error once synchronised:

+Error: arch/arm/dts/imx8mm-venice-gw700x-u-boot.dtsi:26.1-14 Label or
 path pinctrl_pmic not found

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>