BL2 loads secure runtime code(BL3-1, BL3-2) and hence it has to
run in secure world otherwise BL3-1/BL3-2 have to execute from
non-secure memory. Hence, This patch removes the change_security_state()
call in bl1_run_bl2() and replaces it with an assert to confirm
the BL2 as secure.

Fixes ARM-software/tf-issues#314

Change-Id: I611b83f5c4090e58a76a2e950b0d797b46df3c29