Input: add support for Wacom protocol 4 serial tablets

Recent version of xf86-input-wacom no longer support directly accessing
serial tablets. Instead xf86-input-wacom now expects all wacom tablets to
be driven by the kernel and to show up as evdev devices.

This has caused old serial Wacom tablets to stop working for people who still
have such tablets. Julian Squires has written a serio input driver to fix this:
https://github.com/tokenrove/wacom-serial-iv

This is a cleaned up version of this driver with improved Graphire support
(I own an old Graphire myself).

Signed-off-by: Julian Squires <julian@cipht.net>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

diff --cc MAINTAINERS
Simple merge

diff --cc drivers/input/tablet/Kconfig
Simple merge

diff --cc drivers/input/tablet/Makefile
Simple merge

diff --cc drivers/input/tablet/wacom_serial4.c
Simple merge

diff --cc include/uapi/linux/serio.h
Simple merge