ASoC: fsl_aud2htx: select SND_SOC_IMX_PCM_DMA

The newly added driver requires DMA support and fails to build
when that is disabled:

aarch64-linux-ld: sound/soc/fsl/fsl_aud2htx.o: in function `fsl_aud2htx_probe':
fsl_aud2htx.c:(.text+0x3e0): undefined reference to `imx_pcm_dma_init'

Fixes: 8a24c834

 ("ASoC: fsl_aud2htx: Add aud2htx module driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Nicolin Chen <nicoleotsuka@gmail.com>
Link: https://lore.kernel.org/r/20210103135327.3630973-1-arnd@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>