ARM: dts: mvebu: Add device tree for db-88f6820-amc board

This board is a plugin card for some of Marvell's switch development
kits. It's similar to the non-amc board except that it has no SATA
support.

[gregory: fix DTC warning and use the new partition binding]

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