ARM: dts: mvebu: add sdram controller node to Armada-38x

The Armada-38x uses an SDRAM controller that is compatible with the
Armada-XP. The key difference is the width of the bus (XP is 64/32, 38x
is 32/16). The SDRAM controller registers are the same between the two
SoCs.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>