config: add OMX.st.audio_decoder.mp3.mad for Bellagio config

config: add OMX.st.audio_decoder.mp3.mad for Bellagio config

MP3 Software decoder (libmad based) but useful for testing
and to compare with other targets.

GST_OMX_CONFIG_DIR=$HOME/gst/master/gst-omx/config/bellagio/ \
    gst-launch-1.0 filesrc location=mpthreetest.mp3 ! id3demux ! \
    mpegaudioparse ! omxmp3dec ! audioconvert ! pulsesink

Didn't add 'local' in core-name path compared to other components
in that same gstomx.conf file because OMX.st.audio_decoder.mp3.mad
comes with the Ubuntu package 'libomxil-bellagio0-components-mad'.
All other components listed in this gstomx.conf for Bellagio, are
not provided by any Ubuntu packages. It could explain the 'local',
i.e. requiring to build them from source.

dpkg -L libomxil-bellagio0
    /usr/lib/libomxil-bellagio.so.0

https://bugzilla.gnome.org/show_bug.cgi?id=781786

Signed-off-by: Gurkirpal Singh <gurkirpal204@gmail.com>
Signed-off-by: Julien Isorce <jisorce@oblong.com>