doc: board: xilinx: zynq.rst: add description how to flash a SD card

Add a short description in the ZYNQ documentation how to prepare a SD card and
copy the related images to SD card.

Signed-off-by: Johannes Krottmayer <krjdev@gmail.com>
Cc: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>