spi: rspi: Mark OF related data as maybe unused

spi: rspi: Mark OF related data as maybe unused

Driver can match via ID or OF ID table, thus several OF-related methods
will be unused.  Mark the OF structures as __maybe_unused so compiler
can drop them:

  drivers/spi/spi-rspi.c:1203:29: error: ‘qspi_ops’ defined but not used [-Werror=unused-const-variable=]

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230310222857.315629-16-krzysztof.kozlowski@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>