rnndb: update HI and DE state doc from gcx documentation

The gcreg.h from gcx contains quite some register documentation
for HI and DE (2D) state. Integrate this into our xml files
for easier reference and eventual hardware documentation generation.

This may help anyone working on a 2D driver.
fb: implement viv_gpu_top

Displays number of events per second for all
hardware counters in the console. This can be useful to find out
what the GPU is doing.

Still todo: non interactive mode (log statistics to disk), relative
mode (display occupancy), command line parsing, ...