usb: gadget: add isochronous support to gadget zero

Add two isochronous endpoints to the gadget zero source/sink
function. They are enabled by selecting alternate interface 1, so
by default they are not enabled. Module parameters for setting all
the isoc endpoint characteristics are also provided.

Signed-off-by: Pratyush Anand <pratyush.anand@st.com>
Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>