hwmon: (max6639) Use regmap

Add regmap support & remove local caching.

Signed-off-by: Naresh Solanki <naresh.solanki@9elements.com>
Link: https://lore.kernel.org/r/20240503120020.3450972-1-naresh.solanki@9elements.com


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