ANDROID: GKI: Enable CONFIG_USB_PHY for usb drivers like dwc3

The configuration is only selected by other drivers, so add it in
the list of hidden gki configurations. USB_PHY also autoselects
CONFIG_EXTCON, so EXTCON now disappears from gki_defconfig

Bug: 151969174
Test: Builds

Signed-off-by: Sandeep Patil <sspatil@google.com>
Change-Id: I0cc55d2f06fcb5a46daf0578629e1f0a653ac48b