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This patch adds a utility function to return
the address of a symbol. By default, the compiler
generates adr/adrp instruction pair to return
the reference and this utility is used to override
this compiler generated to code and use `ldr`
instruction.

This is needed for Position Independent Executable
when it needs to reference a symbol which is constant
and does not depend on the execute address of the
binary.

For example, on the FVP, the GICv3 register context is
stored in a secure carveout (arm_el3_tzc_dram) within
DDR and does not relocate with the BL image. Now if
BL31 is executing at a different address other than
the compiled address, using adrp/adr instructions to
reference this memory will not work as they generate an
address that is PC relative. The way to get around this
problem is to reference it as non-PC relative (i.e
non-relocatable location) via `ldr` instruction.

Change-Id: I5008a951b007144258121690afb68dc8e12ee6f7
Signed-off-by: Soby Mathew <soby.mathew@arm.com>