km/common: move ivm functions from to ivm.c

The file common.c grows in the past. So move the IVM specific
code into an ivm.c file.

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