usb: gadget: Bind android devices for all UDC gadgets

For targets with multiple UDCs having DRD feature, two UDCs
may be in peripheral mode. In that case, create and bind
android device corresponding to each gadget as and when
mkdir gadget is run. Keep global structure android_device as
android0 (or g1's device) for ease of use by f_midi and
f_audio_source.

Change-Id: Idae6f6d0d8811f27e836f5f6399395a15fbf3c2f
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>