regulator: port over Linux stm32 PWR regulator driver ...

This driver supports internal regulators (1V1, 1V8, 3V3) in the
STMicroelectronics STM32 chips. Control of these regulators will
be required when adding USB support later on.

Imported here is the Linux v5.6-rc1 state of the driver.

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