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After introducing the Mbed TLS shared heap optimisation, reducing BL2
size by 3 pages didn't leave enough space for growth. We give 1 page
back to maximum BL2 size.

Change-Id: I4f05432f00b923693160f69a4e4ec310a37a2b16
Signed-off-by: John Tsichritzis <john.tsichritzis@arm.com>