apalis-imx8: add USBH_EN gpio hog

USB host interface is not working on some Apalis Toradex carrier
boards with Apalis iMX8 SoM. This is due to USBH_EN pin, which
powers USB peripherals, having a strong pull-down on some boards,
and a weak pull-down on the others. This USBH_EN pin is left
unconfigured, which means it is in its default state at cold boot:
input with a strong pull-up. As a result, carrier boards with a
weak pull-down have this signal high enough to trigger power
delivery to USB peripherals, and opposite - boards with strong
pull-down on USBH_EN have this signal below the threshold needed
to trigger USB power delivery.
This change configures the USBH_EN pin as gpio hog, fixing this
issue for all Apalis carrier boards regardless of pull-down
resistor value.

Also, update apalis-imx8_defconfig via savedefconfig.

Signed-off-by: Andrejs Cainikovs <andrejs.cainikovs@toradex.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>