ASoC: mediatek: add MT7622 AFE compatible in documentation

Add support for MT7622 AFE which shares the same binding with MT2701.

Cc: devicetree@vger.kernel.org
Signed-off-by: NRyder Lee <ryder.lee@mediatek.com>
Signed-off-by: NMark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/mt2701-afe-pcm.txt

 Mediatek AFE PCM controller for mt2701
 
 Required properties:
-- compatible = "mediatek,mt2701-audio";
+- compatible: should be one of the followings.
+	      - "mediatek,mt2701-audio"
+	      - "mediatek,mt7622-audio"
 - interrupts: should contain AFE and ASYS interrupts
 - interrupt-names: should be "afe" and "asys"
 - power-domains: should define the power domain