hwmon: (max6639) Add compatible string

Use maxim,max6639 as compatible string for the driver.

Signed-off-by: Naresh Solanki <Naresh.Solanki@9elements.com>
Link: https://lore.kernel.org/r/20230803144401.1151065-2-Naresh.Solanki@9elements.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>