ACPI: thermal: Move to dedicated function sysfs extra attr creation

The ACPI thermal driver creates extra sysfs attributes in its own
directory pointing to the thermal zone it is related to and add a
pointer to the sysfs ACPI thermal device from the thermal zone sysfs
entry.

This is very specific to this ACPI thermal driver, let's encapsulate
the related creation/deletion code to group it inside a function we
can identify later for removal if needed.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
[ rjw: Subject adjustment, removal of trailing white space ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>