ci(coverity): correct flag name in Coverity build

For fvp and juno builds in tf_cov_make the EL3_PAYLOAD_BASE flag was
incorrectly named EL3_PAYLOAD. This patch corrects this to the correct
name.

Signed-off-by: Elizabeth Ho <elizabeth.ho@arm.com>
Change-Id: Ie19d8b658718fcb8cd2bbc6072c2d033a1117bdf
1 file changed