Fix GPIO mux

[npetersen2]

Resolves: #226

Resolves: #230

This PR updates the GPIO subsystem of the REV E firmware design:

• Fixes the bitwidth errors when building the REV E FPGA design in Vivado (reported in: #226)
• Implements the GP3IO mux core logic -- this was not actually implemented before. Whoops. (reported in: #230)
• Updates the code to only allow using the original drv/gpio_mux from REV D hardware, and only use drv/gp3io_mux from REV E hardware. REV E has an extra I/O lane for each GPIO port, thus the new IP core and routing solution.

[npetersen2]

Do not merge yet. I need to test on hardware. Once confirmed working, we will merge to develop then merge to release branch v1.0.x and make a new bug fix release. Yay, first bug fix! :)

[npetersen2]

Tested on hardware REV E, GP3IO mux works.