[TASK] Add Zynq SoC platform documentation

- Add platform documentation for Zynq SoC.
- Update build steps for demos, drivers, hardware and stack
  to compile it for target Xilinx ARM.

Change-Id: I62a2da8fbf8bd11585b840dbec74304e801a5517
Signed-off-by: Joerg Zelenka <joerg.zelenka@br-automation.com>
[TASK] Add known issues about Xilinx Zynq MN demo

Add known issues about Xilinx Zynq MN demo.

Change-Id: Ia19b3702961e9aadc52bef55bae3db56e7b25656
Signed-off-by: Joerg Zelenka <joerg.zelenka@br-automation.com>