Add USB OTG support

USB gadget mode does currently not work OOTB on Armbian, but it can be enabled easily with a device tree change or overlay:

/dts-v1/;
/plugin/;
/ {
	compatible = "radxa,zero", "amlogic,g12a";
	fragment@0 {
		target = <&usb>;
		__overlay__ {
			dr_mode = "otg";
		};
	};
};

I can open a PR for adding this as device tree overlay, or enabling OTG OOTB on Radxa Zero, like vendor did: https://github.com/radxa/kernel/commit/775a467

The latter would be simple. In case of an overlay, I'd need some hint about how to do it best:

• Adding it to the general meson64 overlays patch seems to be right: https://github.com/armbian/build/blob/master/patch/kernel/archive/meson64-6.0/general-meson64-overlays.patch
• But how to prefix or set `compatible` attribute best to assure it can be applied, resp. make clear that it works on Radxa Zero only (as state of testing)?