ASoC: mediatek: Add MTK_STREAM_NUM to mtk-base-afe.h

Add MTK_STREAM_NUM to common header and modify related stuff so that
the other SoCs can reuse it.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Reviewed-by: Garlic Tseng <garlic.tseng@mediatek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>