arm/trusted-firmware-a: correct broken git version

Add UPSTREAM_VERSION_UNKNOWN to avoid the "UNKNOWN_BORKEN" when running
`devtool check-upgrade-status`

Signed-off-by: Jon Mason <jon.mason@arm.com>

meta-arm/recipes-bsp/trusted-firmware-a/trusted-firmware-a_git.bb

@@ -20,3 +20,4 @@ SRC_URI += "\
 
 # Not a release recipe, try our hardest to not pull this in implicitly
 DEFAULT_PREFERENCE = "-1"
+UPSTREAM_CHECK_COMMITS = "1"