imx: Support i.MX6 High Assurance Boot authentication

When CONFIG_SECURE_BOOT is enabled, the signed images
like kernel and dtb can be authenticated using iMX6 CAAM.
The added command hab_auth_img can be used for HAB
authentication of images. The command takes the image
DDR location, IVT (Image Vector Table) offset inside
image as parameters. Detailed info about signing images
can be found in Freescale AppNote AN4581.

Signed-off-by: Nitin Garg <nitin.garg@freescale.com>