mfd: bd9571mwv: Use devm_mfd_add_devices()

To remove mfd devices when unload this driver, should use
devm_mfd_add_devices() instead.

Fixes: d3ea2127 ("mfd: Add ROHM BD9571MWV-M MFD PMIC driver")
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Acked-for-MFD-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>