spi: cadence-qspi: Move ref clock calculation to probe

"assigned-clock-parents" and "assigned-clock-rates" DT properties take
effect only after ofdata_to_platdata() when clk_set_defaults() is called
in device_probe(). Therefore clk get rate() would return a wrong value
in ofdata_to_platdata() when compared with probe. Hence it needs to be
moved to probe.

Tested on u-boot-ti/next.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Acked-by: Vignesh Raghavendra <vigneshr@ti.com>
Acked-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>