ASoC: sun4i-i2s: Update binding documentation to include A31

Add a new compatible for sun4i-i2s driver to handle some
SoCs that have a reset line that must be asserted/deasserted.

This new compatible, "allwinner,sun6i-a31-i2s", requires the
property "resets" which should be a phandle to the reset line.
Except these differences, the compatible is identical to previous one
which will not handle a reset line.
Signed-off-by: Mylène Josserand <mylene.josserand@free-electrons.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/sun4i-i2s.txt

@@ -7,6 +7,7 @@ Required properties:
 
 - compatible: should be one of the followings
    - "allwinner,sun4i-a10-i2s"
+   - "allwinner,sun6i-a31-i2s"
 - reg: physical base address of the controller and length of memory mapped
   region.
 - interrupts: should contain the I2S interrupt.
@@ -19,6 +20,10 @@ Required properties:
    - "mod" : module clock for the I2S controller
 - #sound-dai-cells : Must be equal to 0
 
+Required properties for the following compatibles:
+	- "allwinner,sun6i-a31-i2s"
+- resets: phandle to the reset line for this codec
+
 Example:
 
 i2s0: i2s@01c22400 {