add gpio dump script from wiki

Initially this is a pull request / RFC into our own fork, and if it's useful/appropriate, we can suggest it upstream.

This still generates reports of the form 

   -163: RK30_PIN0_PA3 = 0
   +163: RK30_PIN0_PA3 = 1

Which is, I believe, the names of the defines in the 3.0.x kernels, not the straight banks and numbers of the 3.17 era kernels.  I suggest leaving off that decode change for a later date