imx6: add status reporting for HAB status

Add functions to report the HAB (High Assurance Boot) status
of e.g. i.MX6 CPUs.

This is taken from

git://git.freescale.com/imx/uboot-imx.git branch imx_v2009.08_3.0.35_4.0.0
cpu/arm_cortexa8/mx6/generic.c
include/asm-arm/arch-mx6/mx6_secure.h