arm, keymile: rename MACH_SUEN3 to MACH_KM_KIRKWOOD

The MACH_TYPE SUEN3 is now to specific for keymile boards, because
other boards similar to suen3 will follow. So the MACH_SUEN3 was renamed
to MACH_KM_KIRKWOOD.

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