dt-bindings: input: Convert mpr121 binding to json-schema

Convert the mpr121 binding to DT schema format using json-schema.

Signed-off-by: Michal Vokáč <michal.vokac@ysoft.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>