ASoC: fsl_spdif: Select regmap-mmio

The S/PDIF driver needs regmap so select it to make sure it gets
included in the build.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Acked-by: Nicolin Chen <b42378@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>