hwmon: (adt7x10) Convert to use regmap

Using regmap lets us use the regmap subsystem for SPI vs. I2C register
accesses. It lets us hide access differences in backend code and lets
the common code just access registers without knowing their size.
We can also use regmap for register caching.

Tested-by: Cosmin Tanislav <cosmin.tanislav@analog.com>
Reviewed-by: Cosmin Tanislav <cosmin.tanislav@analog.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>