usb: gadget: zero: Add support for interrupt EP

Interrupt endpoints behave quite similar to the bulk endpoints with the
difference that the endpoints expect data sending/reception request at
particular intervals till the whole data has not been transmitted.

The interrupt EP support is added to gadget zero. A new alternate setting (=2)
has been added. It has 2 interrupt endpoints. The default parameters are set as:
	bInterval: 1 ms for FS and 8 uFrames (implying 1 ms) for HS/SS
	wMaxPacketSize: 64 bytes for FS and 1024 bytes for HS/SS
However, the same can be overridden through the module parameter interface.

The code is tested for HS and SS on a platform having DWC3 controller.

Signed-off-by: Amit Virdi <amit.virdi@st.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>