tty/serial: Add GPIOLIB helpers for controlling modem lines

This patch add some helpers to control modem lines (CTS/RTS/DSR...) via
GPIO.
This will be useful for many boards which have a serial controller that
only handle CTS/RTS pins (or even just RX/TX).

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Tested-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --cc Documentation/serial/driver
Simple merge

diff --cc drivers/tty/serial/Kconfig
Simple merge

diff --cc drivers/tty/serial/Makefile
Simple merge

diff --cc drivers/tty/serial/serial_mctrl_gpio.c
Simple merge

diff --cc drivers/tty/serial/serial_mctrl_gpio.h
Simple merge