usb: gadget: udc: add udc-core from linux kernel to u-boot

usb: gadget: udc: add udc-core from linux kernel to u-boot

Added udc-core.c from linux kernel 3.19-rc1 (97bf6af1f9) to u-boot.
This will be adapted to work with u-boot in the
following patches.
Adding support for udc will help to seamlessly port dwc3 driver from
linux kernel to u-boot (since dwc3 uses udc-core) and it'll also help
to add support for multiple gadget controllers to be functional at the
same time.
All other gadget drivers can also be adapted to use udc-core.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Reviewed-by: Lukasz Majewski <l.majewski@samsung.com>