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

drivers/spi/spi-cadence.c

@@ -101,6 +101,7 @@
  * @regs:		Virtual address of the SPI controller registers
  * @ref_clk:		Pointer to the peripheral clock
  * @pclk:		Pointer to the APB clock
+ * @clk_rate:		Reference clock frequency, taken from @ref_clk
  * @speed_hz:		Current SPI bus clock speed in Hz
  * @txbuf:		Pointer	to the TX buffer
  * @rxbuf:		Pointer to the RX buffer