spi: cadence: mark pm functions __maybe_unused

The newly added runtime PM support for the cadence spi driver
causes harmless warnings when PM is disabled:

drivers/spi/spi-cadence.c:681:12: warning: 'cnds_runtime_suspend' defined but not used
drivers/spi/spi-cadence.c:652:12: warning: 'cnds_runtime_resume' defined but not used

This adds __maybe_unused annotations to the respective functions
to shut up the warnings, while leaving the code in place for
compile testing and avoiding ugly #ifdefs.

Fixes: d36ccd9f ("spi: cadence: Runtime pm adaptation")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Shubhrajyoti Datta <shubhraj@xilinx.com>
Signed-off-by: Mark Brown <broonie@kernel.org>