mfd: bd718x7: simplify by cleaning unnecessary device data

Most ROHM PMIC sub-devices only use the regmap pointer from
parent device. They can obtain this by dev_get_regamap so in
most cases the MFD device does not need to allocate and populate
the driver data. Simplify drivers by removing this.

The BD70528 still needs the access to watchdog mutex so keep
rohm_regmap_dev in use on BD70528 RTC and WDG drivers for now.

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