thermal: provide an UAPI header file

include/linux/thermal.h contains definitions for the Thermal generic
netlink family, but none of the valuable information relevant to
user-space such as the Genl family name, multicast group, version or
command set and data types is exported to user-space.

Export all the relevant generic netlink information to user-space to
make this genl family usable by user-space, and while at it, export
THERMAL_NAME_LENGTH since it limits name length for thermal_hwmon
devices.

Kbuild and MAINTAINERS are also updated accordingly to reflect this new
file: include/uapi/linux/thermal.h.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>