ASoC: mediatek: add documents for btcvsd driver

document for btcvsd driver
Signed-off-by: KaiChieh Chuang <kaichieh.chuang@mediatek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/mtk-btcvsd-snd.txt

+Mediatek ALSA BT SCO CVSD/MSBC Driver
+
+Required properties:
+- compatible = "mediatek,mtk-btcvsd-snd";
+- reg: register location and size of PKV and SRAM_BANK2
+- interrupts: should contain BTSCO interrupt
+- mediatek,infracfg: the phandles of INFRASYS
+- mediatek,offset: Array contains of register offset and mask
+    infra_misc_offset,
+    infra_conn_bt_cvsd_mask,
+    cvsd_mcu_read_offset,
+    cvsd_mcu_write_offset,
+    cvsd_packet_indicator_offset
+
+Example:
+
+	mtk-btcvsd-snd@18000000 {
+		compatible = "mediatek,mtk-btcvsd-snd";
+		reg=<0 0x18000000 0 0x1000>,
+		    <0 0x18080000 0 0x8000>;
+		interrupts = <GIC_SPI 286 IRQ_TYPE_LEVEL_LOW>;
+		mediatek,infracfg = <&infrasys>;
+		mediatek,offset = <0xf00 0x800 0xfd0 0xfd4 0xfd8>;
+	};