boot/ti-k3-r5-loader: new package

This is a separate U-Boot package that allows to build the
intermediate SPL binary for the Cortex-R5 core on TI's k3
platforms. Usually, the resulting SPL will be used for k3-image-gen to
build the full R5 boot binary tiboot3.bin.

Signed-off-by: Xuanhao Shi <x-shi@ti.com>
Signed-off-by: Anand Gadiyar <gadiyar@ti.com>
Reviewed-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Acked-by: Andrew Davis <afd@ti.com>
Tested-by: Bryan Brattlof <bb@ti.com>
Cc: Romain Naour <romain.naour@gmail.com>
Cc: Francois Perrad <francois.perrad@gadz.org>
Reviewed-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>