This patch reduces BL2 size by 12kB. Thanks to the shared Mbed TLS heap
between BL1 and BL2, BL2 now requires less memory since it doesn't need
to allocate a heap anymore.

Change-Id: I58a15f8c424273650c9f55112abe88105b6cdbae
Signed-off-by: John Tsichritzis <john.tsichritzis@arm.com>