power: add FAN53555 family support

This adds a driver for the FAN53555 family of regulators and wraps it
in a PMIC implementation.

While these devices support a 'normal' and 'suspend' mode (controlled
via an external pin) to switch between two programmable voltages, this
incarnation of the driver assumes that the device is always operating
in 'normal' mode.

Only setting/reading the programmed voltage is supported at this time
and the following device functionality remains unsupported:
  - switching the selected voltage (via a GPIO)
  - disabling the voltage output via software-control
This matches the functionality of the Linux driver.

Tested on a RK3399-Q7 (with 'option 5' devices): setting voltages from
the U-Boot shell and verifying output voltages on the board.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: Klaus Goger <klaus.goger@theobroma-systems.com>
diff --git a/drivers/power/regulator/Kconfig b/drivers/power/regulator/Kconfig
index 2561a8a..09b311d 100644
--- a/drivers/power/regulator/Kconfig
+++ b/drivers/power/regulator/Kconfig
@@ -69,6 +69,22 @@
 	features for REGULATOR MAX77686. The driver implements get/set api for:
 	value, enable and mode.
 
+config DM_REGULATOR_FAN53555
+	bool "Enable Driver Model for REGULATOR FAN53555"
+	depends on DM_PMIC_FAN53555
+	help
+	  This config enables implementation of driver-model regulator
+	  uclass features for the FAN53555 regulator. The FAN53555 is
+	  a (family of) single-output regulators that supports
+	  transitioning between two different output voltages based on
+	  an voltage selection pin.
+
+	  The driver implements a get/set api for the voltage of the
+	  'normal mode' voltage only. Switching to 'suspend mode'
+	  (i.e. the alternate voltage), disabling output via software,
+	  or switching the mode is not supported by this driver (at
+	  this time).
+
 config DM_REGULATOR_FIXED
 	bool "Enable Driver Model for REGULATOR Fixed value"
 	depends on DM_REGULATOR