spd: trusty: disable error messages seen during boot

Platforms that do not support Trusty, usually see error
messages from the Trusty SPD, during boot. This can be
interpreted as a boot failure.

This patch lowers the logging level for those error messages
to avoid confusion.

Change-Id: I931baa2c6db0de1aee17383039bc29ed229a1f25
Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>