ARM: mvebu: add audio I2S controller to Armada 370 Device Tree

The Armada 370 SoC has an I2S audio controller. This commit adds the
description of this controller to the Device Tree describing this SoC,
as well as two possible muxing configurations for the I2S bus pins.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>