hwmon: (spd5118) Use regmap to implement paging

Using regmap for paging significantly improves caching since the regmap
cache no longer needs to be cleared after changing the page, so let's
use it.

Suggested-by: Armin Wolf <W_Armin@gmx.de>
Tested-by: Armin Wolf <W_Armin@gmx.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>