spi: zynq_spi: Remove unneeded headers

- Removed unneeded inclusion of header files
- Add "Xilinx" on license text

Cc: Michal Simek <michal.simek@xilinx.com>
Cc: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Jagan Teki <jteki@openedev.com>