plat/arm: enlarge the BL2 size on Arm platforms when TBB is enabled

For Trusted Board Boot, BL2 needs more space to support the ECDSA
and ECDSA+RSA algorithms.

Change-Id: Ie7eda9a1315ce836dbc6d18d6588f8d17891a92d
Signed-off-by: Qixiang Xu <qixiang.xu@arm.com>
3 files changed