USB: gadget: core: create sysfs link between udc and gadget

udc device and gadget device are tightly coupled, yet there's no good
way to corelate the two. Add a sysfs link in udc that points to the
corresponding gadget device.
An example use case: userspace configures a f_midi configfs driver and
bind the udc device, then it tries to locate the corresponding midi
device, which is a child device of the gadget device. The gadget device
that's associated to the udc device has to be identified in order to
index the midi device. Having a sysfs link would make things much
easier.

Signed-off-by: Roy Luo <royluo@google.com>
Link: https://lore.kernel.org/r/20240307030922.3573161-1-royluo@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>