firmware: zynqmp: Add documentation for runtime feature config

Add documentation for sysfs entries like feature_config_id and
 feature_config_value for runtime feature configuration. The support is
 added for an over temperature and external watchdog features.

Acked-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Ronak Jain <ronak.jain@xilinx.com>
Link: https://lore.kernel.org/r/20220209082709.32378-3-ronak.jain@xilinx.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>