dt-bindings: mfd: Document ROHM BD71828 bindings

ROHM BD71828 Power management IC integrates 7 buck converters, 7 LDOs,
a real-time clock (RTC), 3 GPO/regulator control pins, HALL input
and a 32.768 kHz clock gate.

Document the dt bindings drivers are using.

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>