ASoC: atmel-i2s: dt-bindings: add DT bindings for I2S controller

This patch adds DT bindings for the new Atmel I2S controller embedded
inside sama5d2x SoCs.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@microchip.com>
Signed-off-by: Mark Brown <broonie@kernel.org>