hwmon: (it87) Convert to use new hwmon API

Convert to use devm_hwmon_device_register_with_groups to simplify
code and reduce code size. This also attaches sysfs attributes
to the hwmon device and no longer to the platform device.

Tested-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>