arm64: dts: marvell: armada-3720-db: Add information about the V2 board

The initial device tree file was for the board V1.4. Now the V2.0 board
is also available. The same dtb will work for both, but the CON number
have changed, so update the comment in the dts to reflect this.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>