spi: zynq-qspi: Change clock names

spi: zynq-qspi: Change clock names

Use the name pclk instead of aper_clk and change the variable name accordingly.
Change variable name from devclk to refclk just for clarity.
Changes made in driver and relevant dts.

Signed-off-by: Harini Katakam <harinik@xilinx.com>
Reviewed-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>