ASoC: clarify that SND_SOC_IMX_SGTL5000 is the old driver

Both SND_SOC_IMX_SGTL5000 and SND_SOC_FSL_ASOC_CARD implement the
fsl,imx-audio-sgtl5000 compatible string, which is confusing. It took a
little research to find out that the latter is much newer and it is
supposed to be the preferred choice since several years.

Add a clarification note to avoid wasting time for future readers.

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Link: https://lore.kernel.org/r/20230303093410.357621-1-luca.ceresoli@bootlin.com
Signed-off-by: Mark Brown <broonie@kernel.org>