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Following steps are required to boot JUNO in AArch32 state:
1> BL1, in AArch64 state, loads BL2.
2> BL2, in AArch64 state, initializes DDR.
  Loads SP_MIN & BL33 (AArch32 executable)images.
  Calls RUN_IMAGE SMC to go back to BL1.
3> BL1 writes AArch32 executable opcodes, to load and branch
  at the entrypoint address of SP_MIN, at HI-VECTOR address and
  then request for warm reset in AArch32 state using RMR_EL3.

This patch makes following changes to facilitate above steps:
* Added assembly function to carry out step 3 above.
* Added region in TZC that enables Secure access to the
  HI-VECTOR(0xFFFF0000) address space.
* AArch32 image descriptor is used, in BL2, to load
  SP_MIN and BL33 AArch32 executable images.

A new flag `JUNO_AARCH32_EL3_RUNTIME` is introduced that
controls above changes. By default this flag is disabled.

NOTE: BL1 and BL2 are not supported in AArch32 state for JUNO.

Change-Id: I091d56a0e6d36663e6d9d2bb53c92c672195d1ec
Signed-off-by: Yatharth Kochar <yatharth.kochar@arm.com>
Signed-off-by: dp-arm <dimitris.papastamos@arm.com>