ARM: dts: turris-omnia: Define S/PDIF audio card

Turris Omnia has GPIO51 exported on pin header U16, which works in S/PDIF
output mode. So define S/PDIF audio output card for this pin.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>