fpga manager: Add Xilinx slave serial SPI driver

The driver loads FPGA firmware over SPI, using the "slave serial"
configuration interface on Xilinx FPGAs.

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