ARM: mvebu: Add support for NOR flash device on Armada XP-GP board

The Armada XP Development Board DB-MV784MP-GP has a NOR flash device
connected to the Device Bus. This commit adds the device tree node
to support this device.

This SoC supports a flexible and dynamic decoding window allocation
scheme; but since this feature is still not implemented we need
to specify the window base address in the device tree node itself.

This base address has been selected in a completely arbitrary fashion.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>