ACPI: thinkpad-acpi: use a separate platform device for hwmon and name it (v2)

Use a separate platform device and driver ("thinkpad_hwmon") to attach
hwmon attributes and class, and add a name attribute of "thinkpad" to
it, which defines the hwmon device name for libsensors4.

This makes thinkpad-acpi compatible with libsensors4 from lm-sensors, and
the platform driver and device split will make it much easier to separate
hwmon functionality into its own module later on.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Cc: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Len Brown <len.brown@intel.com>