tf-coverity: exclude io_dummy.c file for the coverage

None of the upstream platform using io_dummy.c file hence
exclude it for the coverage.

Change-Id: I8df7d81166451c2a11759c2511ea7f62acd143ea
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
1 file changed