usb: gadget: udc: document our sysfs ABI
usb: gadget: udc: document our sysfs ABI

I noticed that this has been missing for quite
some time so I decided it was about time to
document it.

Signed-off-by: Felipe Balbi <94dddeeef08b001e003cce128ddc162a4e2c6cd2@ti.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs