dt-bindings: clock: mediatek: update audsys documentation to adapt MFD device

The MediaTek audio hardware block that exposes functionalities that are
handled by separate subsystems in the kernel.  These functions are all
mapped somewhere at 0x112xxxxx, and there are some control bits are mixed
up with other functions within the same registers.

This patch modifies example to illustrate child nodes.
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>

Documentation/devicetree/bindings/arm/mediatek/mediatek,audsys.txt

@@ -13,10 +13,19 @@ The AUDSYS controller uses the common clk binding from
 Documentation/devicetree/bindings/clock/clock-bindings.txt
 The available clocks are defined in dt-bindings/clock/mt*-clk.h.
 
+Required sub-nodes:
+-------
+For common binding part and usage, refer to
+../sonud/mt2701-afe-pcm.txt.
+
 Example:
 
-audsys: audsys@11220000 {
-	compatible = "mediatek,mt7622-audsys", "syscon";
-	reg = <0 0x11220000 0 0x1000>;
-	#clock-cells = <1>;
-};
+	audsys: clock-controller@11220000 {
+		compatible = "mediatek,mt7622-audsys", "syscon";
+		reg = <0 0x11220000 0 0x2000>;
+		#clock-cells = <1>;
+
+		afe: audio-controller {
+			...
+		};
+	};