regulator: rk8xx: pass pmic udevice instead of regulator to all internal functions

For the sake of consistency, make all internal (starting with _)
functions expect a pmic udevice instead of a regulator udevice.

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>  # chromebook-bob