Attest: Verify the exposure of shared data to NSPE

There is a shared buffer between bootloader and SPE to exchange
boot data. This check was introduced to verify that a misconfiguration
in the platform layer does not cause that shared buffer is
accidentally exposed to NSPE, therefore risking to tamper sensitive
data.

Change-Id: I1eea2d644545b600c87b9e6199f742f7af9bc421
Signed-off-by: Tamas Ban <tamas.ban@arm.com>