board: qcs404-evb: Enable USB3 specific PMIC GPIO

For USB3 host controller to detect devices on the bus it is required to
enable a PMIC GPIO: usb_vbus_boost_pin. So enable that during board
specific initialization.

And since this PMIC GPIO parsing is quite u-boot specific, so add a
DT override to qcs404-evb-uboot.dtsi to represent usb_vbus_boost_pin.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>