usb: gadget: remove imx_udc

That driver hasn't been really maintained for
a long time. It doesn't compile in any way, it
includes non-existent headers, has no users,
and is just plain broken.

The person who used to work with that driver
has publicly stated that he has no plans to
touch that driver again and is ok with removal[1].

Due to these factors, imx_udc is now removed from
the tree, if someone really believe it needs to
be kept, please fix the bugs in that driver.

[1] http://marc.info/?l=linux-usb&m=136197620417636&w=2

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