How to increase QSPI flash clock frequncy in Zynq ZC702
Check out the xilinx answer 

AR# 46880 Zynq-7000 Example Design - Linear QSPI Performance (Max Effective Throughput)


  This example design shows how to achieve QSPI (in Linear Mode) Maximum
  Effective Throughput with a 100 MHz SPI clock.


There you can get the maximum throughput with the QSPI interface. 

You can find the source code for the project here:
xdmaps_example_w_intr_14_2.c