hwmon: (pmbus/max16601) Determine and use number of populated phases

The MAX16601 can report the number of populated phases. Use this
information to only create sysfs attributes for populated phases.

Cc: Alex Qiu <xqiu@google.com>
Cc: Ugur Usug <Ugur.Usug@maximintegrated.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20210125185327.93282-1-linux@roeck-us.net


Reviewed-by: Alex Qiu <xqiu@google.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>