dt-bindings: clock: Add bindings for ZynqMP clock driver

Add documentation to describe Xilinx ZynqMP clock driver
bindings.

Signed-off-by: Rajan Vaja <rajan.vaja@xilinx.com>
Signed-off-by: Jolly Shah <jollys@xilinx.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>