dt_bindings: mfd: Add ROHM BD71815 PMIC

Document DT bindings for ROHM BD71815.

BD71815 is a single-chip power management IC mainly for battery-powered
portable devices. The IC integrates 5 bucks, 7 LDOs, a boost driver for
LED, a battery charger with a Coulomb counter, a real-time clock, a 32kHz
clock and two general-purpose outputs although only one is documented by
the data-sheet.

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>