ARM: dts: omap4-droid4: Add touchscreen

Droid4's touchscreen can be used with mainline's maxtouch driver. The
touchscreen's lower area is used for four soft buttons (KEY_MENU,
KEY_HOME, KEY_BACK, KEY_SEARCH), but that does not seem to be currently
supported by the mainline kernel.

The mxt224 configuration can be saved with "mxt-app" for the kernel
to load. It can be saved after the first boot with:

# mxt-app -d i2c-dev:1-004a --save /lib/firmware/maxtouch.cfg

Where the mxt-app can be found at:

https://github.com/atmel-maxtouch/mxt-app

The firmware for the droid 4 mxt224 comes with GPLv2 license in the
Motorola Linux kernel sources. This firmware can be dumped out with
"droid4-touchscreen-firmware" program at:

https://github.com/tmlind/droid4-touchscreen-firmware



The related LCD patches are still pending, but when merged,
the touchscreen can be rotated in X with something like:

# xrandr --output DSI-1 --rotate right
# xinput set-prop 6 'Coordinate Transformation Matrix' \
	0 1 0 -1 0 1 0 0 1

For now, we rely on a gpio-hog but later on we can add the reset
gpio handling to the driver and have it load the maxtouch.cfg and
maxtouch.fw on boot.

This patch is based on combined similar patches done by me and
Sebastian.

Signed-off-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>