Updated the Juno platform documentation to reflect testing with SCP
version 2.14.0 instead of 2.15.0. The version used in testing was
reverted in [1] due to instability in the tftf-manual-reboot tests
caused by upgrading to v2.15 and v2.16.
[1] https://review.trustedfirmware.org/c/ci/tf-a-ci-scripts/+/37997
Change-Id: I48c2b51a33950ad096e021d7bdd9cdb6a1303f8c
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>