arm: meson-gx: add board_usb_init()/cleanup() for USB gadget

Add arch code to initialize USB Gadget mode using the DWC2 controller,
and using the previously added set_mode() phy functions.

[narmstrong: fixup board_usb_cleanup call to phy_meson_gxl_usb2_set_mode]
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>