dt-bindings: firmware: xilinx: Describe missing child nodes

dt-bindings: firmware: xilinx: Describe missing child nodes

Firmware node has more than fpga, aes and clock child nodes but also power,
reset, gpio, pinctrl and pcap which are not described yet.
All of them have binding in separate files but there is missing connection
to firmware node that's why describe it.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/1d7988cfadf3554d11f0779f96a670b4fd86ce5a.1703161663.git.michal.simek@amd.com
Signed-off-by: Michal Simek <michal.simek@amd.com>