Boot: Clear RAM before starting secure firmware

Clearing RAM to not leak accidentally any sensitive
information to software components running after boot.
This change also addressing the conformance with
the R30_TBFU_EXEC rule in PSA-TBFU spec (version 1.0.beta.1).

Change-Id: I173ecee9f2c163d385d74c2f14887ed655df7cd5
Signed-off-by: Tamas Ban <tamas.ban@arm.com>