usb: gadget: Add CDC ACM function

Add support for CDC ACM using the new UDC and gadget API. This protocol
can be used for serial over USB data transfer and is widely supported
by various OS (GNU/Linux, MS-Windows, OSX...). The usual purpose of
such link is to access device debug console and can be useful for
products not exposing regular UART to the user.

A default stdio device named 'usbacm' is created, and can be used
to redirect console to USB link over CDC ACM:

> setenv stdin usbacm; setenv stdout usbacm

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>