Add support to expose the PMBUS regulator.

Tested on MP2973 and MP2971.
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Signed-off-by: Naresh Solanki <Naresh.Solanki@9elements.com>
Link: https://lore.kernel.org/r/20230714135124.2645339-7-Naresh.Solanki@9elements.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>