regulator: import linear voltage range helpers ...

The incoming stpmic1 regulator driver makes use of these helpers
internally. Thus port them out of Linux v5.3.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

regulator: Add support for setting regulator's voltage ...

Add code needed to implement regulator_set_voltage(). Currently only
bare minmum needed for ANATOP driver (added in follow up commit) is
supported.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

regulator: Port basic regmap regulator functions ...

In order to be able to port Linux driver relying on this API, port
various regmap related regulator function to Barebox.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>