dt-bindings: BD718x7 - add missing I2C bus properties

The DT example needs #address-cells and #size-cells for I2C bus or
validity checker will generate warnings. Add these properties in
BD71837 and BD71847 binding examples.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Signed-off-by: Rob Herring <robh@kernel.org>