Add support for MP2971 and MP2973, the successor of MP2975.

The major differences are:
 - On MP2973 and MP2971 the Vref cannot be read and thus most of
   the OVP/current calculations won't work.
 - MP2973 and MP2971 also support LINEAR format for VOUT
 - MP2973 and MP2971 do not support OVP2
 - On MP2973 and MP2971 most registers are in LINEAR format
 - The IMVP9_EN bit has a different position
 - Per phase current sense haven't been implemented.

As on MP2975 most of the FAULT_LIMIT and WARN_LIMIT registers
have been redefined and doesn't provide the functionality as
defined in PMBUS spec.

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/20230714135124.2645339-6-Naresh.Solanki@9elements.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>