hwmon: Convert vt8231 to a platform driver

Convert the vt8231 driver from the nonsensical i2c-isa hack to a
regular platform driver.

Signed-off-by: Roger Lucas <roger@planbit.co.uk>
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Mark M. Hoffman <mhoffman@lightlink.com>