Input: ad7879 - add device tree support

Add device tree support for the I2C and SPI variant of AD7879(-1).
This allows to specify the touchscreen controller as a I2C client
node or SPI slave device. Most of the options available in platform
data are also available as device tree properties, the only exception
being GPIO capabilities, which can not be activated through device
tree currently.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>