ASoC: fsl: make sure the regmap-mmio is actually enabled

Since the Freescale SAI, ESAI and SPDIF are using the regmap-mmio,
there needs a select of REGMAP_MMIO in the Kconfig to ensure that
regmap-mmio is actually enabled.

Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>