Since from mx7, we use fixed IVT offset for all boot devices.
Introduce a new configuration CONFIG_IMX_FIXED_IVT_OFFSET for this.

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Signed-off-by: Ye.Li <B37916@freescale.com>
(cherry picked from commit 88e0a355)
(cherry picked from commit 3d52e221)