as checkpatch proposes to use strict_strtoul instead of
simple_strtoul, introduce it.

Ported this function from Linux 2.6.38 commit ID:
521cb40b0c44418a4fd36dc633f575813d59a43d

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