Add support for Rockchip RK860X regulators

Merge series from Cristian Ciocaltea <cristian.ciocaltea@collabora.com>:

This patch series introduces support for the Rockchip RK860X regulators,
while also providing a few fixes and improvements to the existing fan53555
driver.

RK8600/RK8601 are quite similar to the FAN53555 regulators.

RK8602/RK8603 are a bit different, having a wider output voltage
selection range, from 0.5 V to 1.5 V in 6.25 mV steps. They are used
in the Rock 5B board to power the ARM Cortex-A76 cores and the NPU.