spi: cadence: cache reference clock rate during probe

The issue is that using SPI from a callback under the CCF lock will
deadlock, since this code uses clk_get_rate().

Fixes: c474b386

 ("spi: Add driver for Cadence SPI controller")
Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20210114154217.51996-1-alexandru.ardelean@analog.com
Signed-off-by: Mark Brown <broonie@kernel.org>