f_gsi: Add USB full speed descriptors for DPL interface

Currently USB GSI function driver is passing USB high speed related
descriptors as full speed descriptors with USB DPL interface. This
results into USB composite device enumerating with yellow bang when
USB DPL interface is part of it when enumerated USB speed is full
speed. Fix this issue by adding USB full speed descriptors for USB
DPL interface and using same.

Change-Id: I2163020918c5de3210381d4196a3a655ff0cc537
Signed-off-by: Mayank Rana <mrana@codeaurora.org>