mfd: bd9571mwv: Add volatile register to make DVFS work

Because BD9571MWV_DVFS_MONIVDAC is not defined in the volatile table,
the physical register value is not updated by regmap and DVFS doesn't
work as expected. Fix it!

Fixes: d3ea2127 ("mfd: Add ROHM BD9571MWV-M MFD PMIC driver")
Signed-off-by: Dien Pham <dien.pham.ry@renesas.com>
[wsa: rebase, add 'Fixes', reword commit message]
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>