ARM: Reintroduce MACH_TYPE_KM_KIRKWOOD for keymile ARM boards

We got dropped from Linux mach-types.h because of a lack of mainline
support and this is needed since the last Linux/u-boot mach-types
synchro.

This patch also defines CONFIG_MACH_TYPE for all keymile boards, as
this is a mandatory CONFIG for ARM boards now. The initialization
of gd->bd->bi_arch_number is removed form km_arm.c, our board file.

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