dt: bindings: fpga: add xilinx slave-serial binding description

Add dt binding documentation details for Xilinx FPGA configuration
over slave serial interface.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Acked-by: Moritz Fischer <mdf@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Alan Tull <atull@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>