mfd: bd718x7: Add compatible for BD71850

ROHM BD71850 PMIC is almost identical to BD71847. Main difference is some
initial voltage values for regulators. The BD71850 can be handled by
BD71847 driver but adding own compatible makes it clearer for one who
creates the DT for board containing this PMIC and allows SW to be
differentiating PMICs if needed.

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