fix(scp): revert "chore: bump SCP version to v2.16"

This reverts commit 669cfcf028aec36d0d32057637a6cc6d02781183.

Reason for revert: the SCP image size got increased making a number of
Juno test configs failing to boot.

Change-Id: I43a4d0cb7180bb98100ccabbcf8665dba9a750e7
Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
diff --git a/utils.sh b/utils.sh
index f3db0da..d05ad75 100644
--- a/utils.sh
+++ b/utils.sh
@@ -578,7 +578,7 @@
 css_downloads="${css_downloads:-$tfa_downloads/css}"
 
 # SCP/MCP release binaries.
-scp_mcp_downloads="${scp_mcp_downloads:-$tfa_downloads/css_scp_2.16.0}"
+scp_mcp_downloads="${scp_mcp_downloads:-$tfa_downloads/css_scp_2.15.0}"
 
 linaro_2001_release="${linaro_2001_release:-$tfa_downloads/linaro/20.01}"
 linaro_release="${linaro_release:-$linaro_2001_release}"