mplayer: add missing reverse dependencies for nios2 and sh2a

Propagate to comment and libplayer unsupported platforms bfin and sh2a.

Fixes:
http://autobuild.buildroot.net/results/5d8/5d80ade791319b6a66ac1c342f7b1d25c023f0af/



Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>