ARM: mvebu: enable S/PDIF audio in Armada 370 DB Device Tree

In addition to the analog audio input and output, the Armada 370 DB
also has S/PDIF input and output optical connectors. This commit
improves the Device Tree description of the Armada 370 DB platform to
enable the S/PDIF support.

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