ARM: mvebu: rename DT machine structure for Armada 370/XP

Due to a mistake made when merging Armada 370 and Armada XP DT machine
structures, the name of the structure was incorrectly chosen as being
ARMADA_XP_DT, while the structure also covers Armada 370. Therefore,
we rename the structure to ARMADA_370_XP_DT.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>