docs: dts: Added documentation for Xilinx Zynq Reset Controller bindings.

Signed-off-by: Moritz Fischer <moritz.fischer@ettus.com>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Sören Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>