Change default pin assignment in dmx.c to accomodate original beaglebone and newer black.

Change default pin assignment in dmx.c to accomodate original beaglebone and newer black.
Changed from header P8 pin 3 to P8 pin 12. The new beaglebone black has a emmc dependency that made pin 3 unusable. README updated to include new uio_pruss enable information and additional notes as well.
This change makes it so any new DMX capes created would work on both beaglebones.