dt_bindings: regulator: Add ROHM BD71815 PMIC regulators

Add binding documentation for regulators on ROHM BD71815 PMIC.
5 bucks, 7 LDOs and a boost for LED.

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