ASoC: tas571x: new chip added into TAS571x binding

This adds the TAS5721 into the TAS571x binding.
Signed-off-by: NPetr Kulhavy <petr@barix.com>
Reviewed-by: NKevin Cernekee <cernekee@chromium.org>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NMark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/tas571x.txt

-Texas Instruments TAS5711/TAS5717/TAS5719 stereo power amplifiers
+Texas Instruments TAS5711/TAS5717/TAS5719/TAS5721 stereo power amplifiers
 
 The codec is controlled through an I2C interface.  It also has two other
 signals that can be wired up to GPIOs: reset (strongly recommended), and
@@ -6,7 +6,11 @@ powerdown (optional).
 
 Required properties:
 
-- compatible: "ti,tas5711", "ti,tas5717", or "ti,tas5719"
+- compatible: should be one of the following:
+  - "ti,tas5711",
+  - "ti,tas5717",
+  - "ti,tas5719",
+  - "ti,tas5721"
 - reg: The I2C address of the device
 - #sound-dai-cells: must be equal to 0
 
@@ -25,6 +29,8 @@ Optional properties:
 - PVDD_B-supply: regulator phandle for the PVDD_B supply (5711)
 - PVDD_C-supply: regulator phandle for the PVDD_C supply (5711)
 - PVDD_D-supply: regulator phandle for the PVDD_D supply (5711)
+- DRVDD-supply: regulator phandle for the DRVDD supply (5721)
+- PVDD-supply: regulator phandle for the PVDD supply (5721)
 
 Example: