package/gst-fsl-plugins: add default platform

Set the default platform for gst-fsl-plugins, based on the one choosen
as the generic Freescale i.MX platform.

We can't use only this generic platform choice, as gst-fsl-plugins has
support for more platforms.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Gary Bisson <bisson.gary@gmail.com>
Cc: Hadrien Boutteville <hadrien.boutteville@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>