USB: android gadget: Add interrupt ep and modem support in f_serial

Add interrupt endpoint support to send modem notifications to USB Host.

Add communication class descriptors to enumerate the function as a modem

Add Communication device class requests to configure modem line
settings.

As a part of call management, device needs to send modem events
(Ring Indicator and Carrier Detect) to host through interrupt
endpoint. Add support for Ring Indicator and Carrier Detect events
in f_serial.

Similarly, route modem control information (DTR and RTS) to upper
layers through TTY tiocmget API in f_serial.

provide CONFIG option to enable generic serial as a part of android
gadget.

provide CONFIG option to include MODEM functionality in the generic
serial driver.

Signed-off-by: Jonnala, Anji <anjir@qualcomm.com>