arm/km: add support for km_kirkwood_128m16 board

The board is similar to the standard km_kirkwood board. From a
u-boot point of view, the only difference is an increased
256 MiB DRAM (128M16). A board based on this design is for
example the SUP12.

Signed-off-by: Karlheinz Jerg <karlheinz.jerg@keymile.com>
Signed-off-by: Holger Brunck <holger.brunck@keymile.com>