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>