arm: am4x: add U-Boot FIT signing and SPL image post-processing

Modify the SPL build procedure for AM437x high-security (HS) device
variants to create a secure u-boot_HS.img FIT blob that contains U-Boot
and DTB artifacts signed (and optionally encrypted) with a TI-specific
process based on the CONFIG_TI_SECURE_DEVICE config option and the
externally-provided image signing tool.

Also populate the corresponding FIT image post processing call to be
performed during SPL runtime.

Signed-off-by: Madan Srinivas <madans@ti.com>
Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>