When BL2_AT_EL3 option is enabled some platforms are going to
need a resident part in BL2 because the boot rom may jump to it
after a reset. This patch introduces __TEXT_RESIDENT_START__ and
__TEXT_RESIDENT_END__ linker symbols that mark the resident region.

Change-Id: Ib20c1b8ee257831bcc0ca7d3df98d0cb617a04f8
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>