Add documentation about plat_try_next_boot_source to bl1_platform_setup

Add documentation about plat_try_next_boot_source to bl1_platform_setup

If boot redundancy is required in BL1 then the initialization
of the boot sequence must be done in bl1_platform_setup. In BL2,
we had to add a new function, bl2_preload_setup, because
bl2_platform_setup is called after the images are loaded, making it
invalid for the boot sequence initialization.

Change-Id: I5c177ff142608ed38b4192288b06614343b2b83b
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>