dt-bindings: trivial-devices: Add mcp342x ADCs and drop separate binding doc.

These i2c devices have simple bindings, well described by trivial-device.yaml
so rather than convert the binding doc to yaml, let us just add them to
trivial devices and drop the old binding document.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Link: https://lore.kernel.org/r/20200809111753.156236-3-jic23@kernel.org