arm: xilinx: Add missing dual parallel flash description

Describe flash memories based on the latest DT binding.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/cddf2909d0445eba08b998d42ffc31c1fa3132b9.1698045694.git.michal.simek@amd.com