spi: cadence_qspi: Better debug information on the SPI clock rate

Show what the output clock rate actually is.

Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>