spi: cadence_qspi: Fix CS timings

spi: cadence_qspi: Fix CS timings

The Cadence QSPI controller has specified overheads for the various CS
times that are in addition to those programmed in to the Device Delay
register. The overheads are different for the delays.

In addition, the existing code does not handle the case when the delay
is less than a SCLK period.

This change accurately calculates the additional delays in Ref clocks.

Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>