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Update firmware-design.rst, porting-guide.rst and user-guide.rst
with the information about BL2 at EL3. Firmware-design.rst is
also update to explain how to test this feauture with FVP.

Change-Id: I86d64bc64594e13eb041cea9cefa3f7f3fa745bd
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>