spi: zynq_qspi: Changes to improve qspi performance

The below changes are made to improve the qspi performance
- Use Manual chipselect and auto start
- Fill the TxFIFO to full and use threshold
  value for reception.
- Tested the changes by taking timestamps for reading 1MB data with
  these changes and with old driver. The time taken with  these
  changes is 69ms and with old driver it was 122ms.

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