ARM: mvebu: Add spdif-pins mpp pins for Armada 38x

S/PDIF function on Armada 38x uses only mpp51 pin. So add spdif-pins mpp
pins section for it. It is needed for boards without i2s.

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