Docs: Update TF-M repository reference

The TF-M repository has been moved from
  git.trustedfirmware.org/trusted-firmware-m to
  git.trustedfirmware.org/TF-M/trusted-firmware-m

This patch updates references to the documentation
to reflect that change.

Signed-off-by: Minos Galanakis <minos.galanakis@arm.com>

Change-Id: Ib7d4d45bae7790bf69fcad4fd16f558716b20d92