imx: ventana: enable dm support for USB

Enable dm support for USB (which also requires dm support for fixed
regulators used for vbus enable) and remove usb iomux which is no
longer needed.

We can remove the handling of otgpwr_en gpio as this is defined in
dt as usbotg vbus-supply but we need to keep the handling of
USB_HUB_RST# for boards that have a USB HUB as that isn't defined in
the dt's currently.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>