ARM: kirkwood: select dtbs based on SoC

To prevent problems later with mvebu_v5_defconfig builds, we restrict
the scope of the changes this series makes.

Selecting ARCH_KIRKWOOD and MACH_KIRKWOOD is necessary during the
migration from mach-kirkwood to mach-mvebu.  Until the last four legacy
kirkwood board files have a DT equivalent, we need to support building
DT from both mach-kirkwood and mach-mvebu.

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