Documentation: fpga: dfl: add descriptions for thermal/power management interfaces

This patch add introductions to thermal/power interfaces. They are
implemented as hwmon sysfs interfaces by thermal/power private
feature drivers.

Signed-off-by: Xu Yilun <yilun.xu@intel.com>
Signed-off-by: Wu Hao <hao.wu@intel.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Moritz Fischer <mdf@kernel.org>