Input: add common DT binding for touchscreens

Add common DT binding documentation for touchscreen devices and
implement input_parse_touchscreen_of_params, which parses the common
properties and configures the input device accordingly.

The method currently does not interpret the axis inversion properties,
since there is no matching flag in the generic linux input device.

Reviewed-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
diff --git a/drivers/input/touchscreen/Kconfig b/drivers/input/touchscreen/Kconfig
index 43389c0..8e07fe8 100644
--- a/drivers/input/touchscreen/Kconfig
+++ b/drivers/input/touchscreen/Kconfig
@@ -11,6 +11,10 @@
 
 if INPUT_TOUCHSCREEN
 
+config OF_TOUCHSCREEN
+	def_tristate INPUT
+	depends on INPUT && OF
+
 config TOUCHSCREEN_88PM860X
 	tristate "Marvell 88PM860x touchscreen"
 	depends on MFD_88PM860X