user_guide: Update prerequisite of TF-A version

The change related to enable larger size for the Juno platform
has been recently merged.
Remove the limitation for the SCP-firmware to be built in the
release mode.

Change-Id: If2d1c3c28224c77c9cce0c16495f910d34dec8d1
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>