i2c: synquacer: mark OF related data as maybe unused

The driver can be compile tested with !CONFIG_OF making certain data
unused:

  drivers/i2c/busses/i2c-synquacer.c:632:34: error: ‘synquacer_i2c_dt_ids’ defined but not used [-Werror=unused-const-variable=]

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>