dt-bindings: input: adc-keys: clarify description

dt-bindings: input: adc-keys: clarify description

The current description of ADC keys is not precise enough.

"when this key is pressed" leaves it open if a key is considered pressed
below or above the threshold. This has led to confusion:
drivers/input/keyboard/adc-keys.c ignores the meaning of thresholds and
sets the key that is closest to press-threshold-microvolt.

This patch nails down the definitions and provides an interpretation of the
supplied example.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201222110815.24121-1-xypron.glpk@gmx.de
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>