board/freescale/common: secure_boot: Set bootdelay to -2.

Uboot prompt must not be available while running
secure boot. TO ensure this  bootdelay must be set
to -2.

Signed-off-by: Udit Agarwal <udit.agarwal@nxp.com>
Reviewed-by: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>