dt-bindings: bd718x7: Yamlify and add BD71850

Convert ROHM bd71837 and bd71847 PMIC binding text docs to yaml. Split
the binding document to two separate documents (own documents for BD71837
and BD71847) as they have different amount of regulators. This way we can
better enforce the node name check for regulators. ROHM is also providing
BD71850 - which is almost identical to BD71847 - main difference is some
initial regulator states. The BD71850 can be driven by same driver and it
has same buck/LDO setup as BD71847 - add it to BD71847 binding document and
introduce compatible for it.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>