arm64: dts: verdin-imx8mp: improve pinctrl for vbus-supplies

As we are using two fixed regulators for Verdin USB_1_EN (SODIMM 155)
and Verdin USB_2_EN (SODIMM 185), those should be muxed as GPIOs rather
than OTG_PWR.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>