This adds support for the Delta DPS-920AB PSU.

Only missing feature is fan control which the PSU supports.
Signed-off-by: Robert Marko <robert.marko@sartura.hr>
Link: https://lore.kernel.org/r/20210607103431.2039073-1-robert.marko@sartura.hr
[groeck: Add MODULE_IMPORT_NS(PMBUS);]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>