usb: dwc3: meson-g12a: refactor usb init

Refactor the USB init code patch to handle the Amlogic GXL/GXM needing
to initialize the OTG port as Peripheral mode for the DWC2 IP to probe
correctly.

A secondary, post_init callback is added to setup the OTG PHY mode after
powering up the PHYs and before probing the DWC2 and DWC3 controllers.

Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>