juno: Revert FWU update detect mechanism

The patch 7b56928a unified the FWU mechanism on FVP and Juno
platforms due to issues with MCC firmware not preserving the
NVFLAGS. With MCCv150 firmware, this issue is resolved. Also
writing to the NOR flash while executing from the same flash
in Bypass mode had some stability issues. Hence, since the
MCC firmware issue is resolved, this patch reverts to the
NVFLAGS mechanism to detect FWU. Also, with the introduction
of SDS (Shared Data Structure) by the SCP, the reset syndrome
needs to queried from the appropriate SDS field.

Change-Id: If9c08f1afaaa4fcf197f3186887068103855f554
Signed-off-by: Sathees Balya <sathees.balya@arm.com>
Signed-off-by: Soby Mathew <Soby.Mathew@arm.com>