board: am33xx-hs: Allow post-processing of FIT image on AM33xx

When CONFIG_FIT_IMAGE_POST_PROCESS or CONFIG_SPL_FIT_IMAGE_POST_PROCESS
is enabled board_fit_image_post_process will be called, add this
function to am33xx boards when CONFIG_TI_SECURE_DEVICE is set to
verify the loaded image.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Lokesh Vutla <lokeshvutla@ti.com>