usb: serial: Add support for circuit-switched video telephony driver

circuit switched video telephony driver provides bridge functionality
between mdm9x15 and application in userspace to send and receive control
information and data. It uses generic serial driver which exposes tty
interface to userspace.

Change-Id: I567d3ea971f3fd2f36a071d2639ee2b43b36e233
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>