thermal: add available policies sysfs attribute

The Linux thermal framework support to change thermal governor
policy in userspace, but it can't show what available policies
supported.

This patch adds available_policies attribute to the thermal
framework, it can list the thermal governors which can be
used for a particular zone. This attribute is read only.

Signed-off-by: Wei Ni <wni@nvidia.com>
Reviewed-by: Javi Merino <javi.merino@arm.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>