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This patch reworks the handover interface from: BL1 to BL2 and
BL2 to BL3-1. It removes the raise_el(), change_el() and
run_image() functions as they catered for code paths that were
never exercised. BL1 calls drop_el() to jump into BL2 instead of
doing the same by calling run_image(). Similarly, BL2 issues the
SMC to transfer execution to BL3-1 through BL1 directly. Only x0
and x1 are used to pass arguments to BL31. These arguments and
parameters for running BL3-1 are passed through a reference to a
'el_change_info_t' structure. They were being passed value in
general purpose registers earlier.

Change-Id: Id4fd019a19a9595de063766d4a66295a2c9307e1