power: add FAN53555 family support

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>