arm: mvebu: don't list all boards in dt compat field for Armada 370/XP

Instead of listing explicitly all boards in the .dt_compat field of
the DT_MACHINE_START structure for Armada 370/XP, use instead a
compatible string that is common to all boards using the Armada
370/XP.

This allows to add new boards by just using a different Device Tree,
without having to modify the source code of the kernel.

Note that the name of the array containing the compatible string is
also renamed, to reflect the fact that it no longer contains the list
of all boards.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Tested-by: Gregory CLEMENT <gregory.clement@free-electrons.com>