usb: gadget: dfu: add functional descriptor in descriptor set

The "DFU descriptor set" must contain the "DFU functional descriptor"
but it is missing today in U-Boot code
(cf: DFU spec 1.1, chapter 4.2 DFU Mode Descriptor Set)
This patch only allocate buffer and copy DFU functional descriptor
after interfaces.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay73@gmail.com>