ARM: kirkwood: remove redundant DT board files

With DT support for mv643xx_eth board specific init for some boards now
is unneccessary. Remove those board files, Kconfig entries, and
corresponding entries in kirkwood_defconfig.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>