TTY: tty_port, add some documentation

I forgot to document tty_port_register_device and tty_port_install
when they were added. Fix it now.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_port.c

@@ -33,6 +33,17 @@ void tty_port_init(struct tty_port *port)
 }
 EXPORT_SYMBOL(tty_port_init);
 
+/**
+ * tty_port_register_device - register tty device
+ * @port: tty_port of the device
+ * @driver: tty_driver for this device
+ * @index: index of the tty
+ * @device: parent if exists, otherwise NULL
+ *
+ * It is the same as tty_register_device except the provided @port is linked to
+ * a concrete tty specified by @index. Use this or tty_port_install (or both).
+ * Call tty_port_link_device as a last resort.
+ */
 struct device *tty_port_register_device(struct tty_port *port,
 		struct tty_driver *driver, unsigned index,
 		struct device *device)
@@ -422,6 +433,16 @@ void tty_port_close(struct tty_port *port, struct tty_struct *tty,
 }
 EXPORT_SYMBOL(tty_port_close);
 
+/**
+ * tty_port_install - generic tty->ops->install handler
+ * @port: tty_port of the device
+ * @driver: tty_driver for this device
+ * @tty: tty to be installed
+ *
+ * It is the same as tty_standard_install except the provided @port is linked
+ * to a concrete tty specified by @tty. Use this or tty_port_register_device
+ * (or both). Call tty_port_link_device as a last resort.
+ */
 int tty_port_install(struct tty_port *port, struct tty_driver *driver,
 		struct tty_struct *tty)
 {