arm/km: replace suenx targets with km_kirkwood

suen3 and suen8 were in first HW version quite different, but
now they are from a u-boot point of view similar. So these
two boards can use the same header file. Other keymile boards
differ only in the usage of the PCI interface. Therefore
a target km_kirkwood_pci was introduced. All targets use
the same header file.

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Prafulla Wadaskar <prafulla@marvell.com>
cc: Heiko Schocher <hs@denx.de>