Acknowledge that FWU tests do not work on AEM FVP revC

Mention this as a known issue in the change log.

Also update the run_fwu_fvp.sh script to invoke the revB version
of the FVP rather than the revC.

Change-Id: I58957f4ac3ee37e054fe33bfb246c6f6fc689fdd
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>