spi: Add zynq spi controller driver

Zynq spi controller driver supports 2 buses and
3 chipselects on each bus.

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>