Add symlinks for BBB bus interfaces

Created by: lorforlinux

I have included symlinks for the bus interfaces listed in BeagleBone cape interface spec. I have also updated all the entries on the elinux page except for the SPI, I am not entirely sure how to assign symlinks which correspond to different chip select pins.

@RobertCNelson @jadonk please review the pull request and let me know how to resolve the SPI symlinks.