Input: edt-ft5x06 - increase allowed data range for threshold parameter

The datasheet and application note does not mention an allowed range for
the M09_REGISTER_THRESHOLD parameter. One of our customers needs to set
lower values than 20 and they seem to work just fine on EDT EP0xx0M09 with
T5x06 touch.

So, lacking a known lower limit, we increase the range for thresholds,
and set the lower limit to 0. The documentation is updated accordingly.

Signed-off-by: Schoefegger Stefan <stefan.schoefegger@ginzinger.com>
Signed-off-by: Manfred Schlaegl <manfred.schlaegl@ginzinger.com>
Signed-off-by: Martin Kepplinger <martin.kepplinger@ginzinger.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>