ARM: mvebu: select dtbs from MACH_ARMADA_*

With kirkwood migrating into mach-mvebu, mvebu_v5_defconfig needs to
select ARCH_MVEBU.  Unfortunately, this means that when building a v5
kernel, we unnecessarily build dtbs for the armada v7 boards.

To fix this, we instead select based on MACH_ARMADA_* on a per SoC basis.

Reported-by: Kevin Hilman <khilman@linaro.org>
Acked-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>