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For the BL2_AT_EL3 configuration, move BL2 higher up to make more
space for BL31.  Adjust the BL31 limit to be up to BL2 base.  This is
because BL2 is always resident for the BL2_AT_EL3 configuration and
thus we cannot overlay it with BL31.

Change-Id: I71e89863ed48f5159e8b619f49c7c73b253397aa
Signed-off-by: Dimitris Papastamos <dimitris.papastamos@arm.com>