dt-bindings: mfd: Document first ROHM BD70528 bindings

Document bindings for regulators (3 bucks, 3 LDOs and 2 LED
drivers) and 4 GPIO pins which can be configured for I/O or
as interrupt sources withe configurable trigger levels.

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