watchdog: orion: prepare new Dove DT Kconfig variable

DT-enabled Dove will move over from ARCH_DOVE in mach-dove to MACH_DOVE in
mach-mvebu. As non-DT ARCH_DOVE will stay to rot for a while, add a new
DT-only MACH_DOVE Kconfig.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>