arm/km: add support for kmsuv31 board

This board is from a u-boot point of view a mixture between kmnusa and
a standard km_kirkwood board. We have our u-boot environment in the spi
NOR flash, but we have a direct connection between the kirkwood and the
piggy. A FPGA is connected via the PCIe interface. So we only have to
select the specific features in the board setup.

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
cc: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Prafulla Wadaskar <prafulla@marvell.com>
Acked-By: Prafulla Wadaskar <prafulla@marvell.com>