dt-bindings: input: ads7846: fix property description

The ti,y-max is used for the maximum value of the Y axis.

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>