image: add IH_OS_ARM_TRUSTED_FIRMWARE for ARM Trusted Firmware

To boot on ARMv8 systems with ARM Trusted Firmware, we need to
assemble an ATF-specific parameter structure and also provide the
address of the images started by ATF (e.g. BL3-3, which may be the
full U-Boot).

To allow us to identify an ARM Trusted Firmware contained in a FIT
image, this adds the necessary definitions.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>