spi: spi-cadence: Add missing kernel doc for clk_rate in cdns_spi

Add the missing kernel documentation to silence the build warning.

Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20230523090124.3132-1-ckeepax@opensource.cirrus.com


Signed-off-by: Mark Brown <broonie@kernel.org>