doc/user-guide: custom instructions for TC

Adding custom instructions for TC2 to make it work with the latest
SCP.

As TC release uses an old version of SCP before the directory
restructure. Hence, some modification has to be applied to TC
scp build script.

Along with instructions to rebuild only SCP-firmware and all it's
dependencies.

Signed-off-by: Mohamed Omar Asaker <mohamed.omarasaker@arm.com>