package/ffmpeg: re-enable for mips64r2 and mips64r6
ffmpeg 3 fixed support for mips64r2 and mips64r6, for details see
http://patchwork.ozlabs.org/patch/626756/
Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>