hwmon: (jc42) Convert register access and caching to regmap/regcache

Switch the jc42 driver to use an I2C regmap to access the registers.
Also move over to regmap's built-in caching instead of adding a
custom caching implementation. This works for JC42_REG_TEMP_UPPER,
JC42_REG_TEMP_LOWER and JC42_REG_TEMP_CRITICAL as these values never
change except when explicitly written. The cache For JC42_REG_TEMP is
dropped (regmap can't cache it because it's volatile, meaning it can
change at any time) as well for simplicity and consistency with other
drivers.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Link: https://lore.kernel.org/r/20221023213157.11078-2-martin.blumenstingl@googlemail.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>