Input: edt-ft5x06 - add offset support for ev-ft5726

Unfortunately the evervision focaltech implementation uses two offset
registers, one for the x coordinate and one for y.

This patch extends the driver to handle those offset registers only for
devices that support these.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>