spl: atf: introduce spl_invoke_atf and make bl31_entry private

This adds a new interface spl_invoke_atf() that takes a spl_image_info
argument and then derives the necessary parameters for the ATF entry.
Based on the additional information recorded (into /fit-images) from
the FIT loadables, we can now easily locate the next boot stage.

We now pass a pointer to a FDT as the platform-specific parameter
pointer to ATF (so we don't run into the future headache of every
board/platform defining their own proprietary tag-structure), as
FDT access is already available in ATF.

With the necessary infrastructure in place, we can now update the
support for the ARM Trusted Firmware to dispatch into the
spl_invoke_atf function only if a IH_OS_ARM_TRUSTED_FIRMWARE image is
loaded.

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