GPIO Bit-Bang Speed Increase

@JerryChang

Using the gpio_set_value() APIs in a kernel module. I still observe that toggling the pins on the TX2 is much slower than on the TX1. Additionally the APIs appear to be slower than userspace mmap. It would be useful to know what has changed between TX1 and TX2 gpios and whether there is a better way to toggle gpios quickly or that we are actually limited to the speed observed.