arm: mach-omap2: Flush cache after FIT post-processing image

After we authenticate/decrypt an image we need to flush the caches
as they may still contain bits of the encrypted image. This will
cause failures if we attempt to jump to this image.

Reported-by: Yogesh <Siraswar&lt;yogeshs@ti.com>
Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>