regulator: pbias: program pbias register offset in pbias driver

Add separate compatible strings for every platform and populate the
pbias register offset in the driver data.
This helps avoid depending on the dt for pbias register offset.

Also update the dt binding documentation for the new compatible
strings.

Suggested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Mark Brown <broonie@kernel.org>