USB: gadget: g_multi: added documentation and INF files

A short documentation of the g_multi driver along with INF
files for Windows XP SP3 are provided.

Signed-off-by: Michal Nazarewicz <m.nazarewicz@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>