usb: gadget: Gadget directory cleanup - group UDC drivers

The drivers/usb/gadget directory contains many files.
Files which are related can be distributed into separate directories.
This patch moves the UDC drivers into a separate directory.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>