zynq: Enable CONFIG_ZYNQ_SPI

Tested spi on zynq board with sst flash by enabling
CONFIG_ZYNQ_SPI.

sf probe 1:1 0 0
SF: Detected SST25WF080 with page size 4 KiB, total 1 MiB

Signed-off-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>
Acked-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>