ARM: mvebu: Rename the ARCH_MVEBU menu option

The previous name "Marvell SOCs with Device Tree support" is a bit
ambiguous and not too informative for users. Instead, use a more
appropriate name.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>