ASoC: jz4740: Add binding documentation for jz4740-i2s

This patch adds binding for the jz4740-i2s driver.

Signed-off-by: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>