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 --git a/drivers/tty/serial/Kconfig b/drivers/tty/serial/Kconfig
index d88c058..4bf6d22 100644
--- a/drivers/tty/serial/Kconfig
+++ b/drivers/tty/serial/Kconfig
@@ -1553,4 +1553,7 @@
 
 endmenu
 
+config SERIAL_MCTRL_GPIO
+	tristate
+
 endif # TTY