dt-bindings: input: Add reset-time-sec common property

Add a new common property 'reset-time-sec' to be used in conjunction
with the devices supporting the key pressed reset feature.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>