arm64: mvebu: use single defconfig for Armada8K development boards

arm64: mvebu: use single defconfig for Armada8K development boards

Currently, Marvell Armada8k development board use 3 different
defconfigs:
        mvebu_db-88f7040-nand_defconfig
        mvebu_db-88f7040_defconfig
        mvebu_db-88f8040_defconfig
Having 3 different defconfigs makes maintenance difficult.

This patch removes the defconfigs mentioned above and introduce
a new defconfig which represents the Armada8k family.

NOTE:
In order not to break automatic tools compilation,
a default device-tree is set in the defconfig (armada-8040-db).
However, when compiling u-boot, the user MUST explicitly export
the DEVICE_TREE for the requested board because using A80x0
device-tree on A70x0 might break the device.

For more information, refer to "doc/README.marvell" (intoduced
in this patch).

Change-Id: I98515b6306498358f3722ecf7ac4c87f236ebbd8
Signed-off-by: Igal Liberman <igall@marvell.com>
Signed-off-by: Stefan Roese <sr@denx.de>