msm: mhi_dev: Add AT/DUN command support in UCI driver

This change adds support to send and receive the AT/DUN serial line
state changes to and from the host. Clients can get and set the serial
line state via IOCTL calls.

Signed-off-by: Siva Kumar Akkireddi <sivaa@codeaurora.org>
Change-Id: Ie9f34275844216e1de0212898a85ccb10453fd5b