mplayer: not supported on or1k

MPlayer doesn't support OpenRISC:

The architecture of your CPU (or1k) is not supported by this configure script
It seems nobody has ported MPlayer to your OS or CPU type yet.

so disable it on this architecture.

Fixes:

  http://autobuild.buildroot.net/results/47207dfd10ff6e5ec4ccdcf8454aaf5f408ad1e3/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>