hwmon: (pmbus/mp2975) Prepare for MP2973 and MP2971

Add support for differntiating between the chips.
The following commits will make use of this mechanism.

Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Signed-off-by: Naresh Solanki <Naresh.Solanki@9elements.com>
Link: https://lore.kernel.org/r/20230714135124.2645339-3-Naresh.Solanki@9elements.com


[groeck: double-cast of_device_get_match_data() to make gcc happy]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>