dt-bindings: xilinx: Separate clock binding from firmware doc

Clock description is part of firmware doc. Move clock description
in separate doc.

Signed-off-by: Rajan Vaja <rajan.vaja@xilinx.com>
Signed-off-by: Jolly Shah <jollys@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>