usb: gadget: android: Add serial function support in 2nd config

Serial function driver is used for DUN/MODEM and NMEA use-cases
and it uses various transports as well e.g. tty, smd and
char-bridge. Current design allow Serial function to be present
only in one configuration at a time. Add another instance of
serial function that re-uses transports from first instance
of serial instance and allows to have serial function in
second configuration as well.

Change-Id: I09340ea04da32c7935462295f302a8d4a520b4da
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>