You will find the sources for this "blink" project on my Github page:
A quick note first. I had to perform some trial and error experiments to find out the actual GPIO signal this was controlled by. The schematics have a typo that indicates B5 rather than B3, although if you trace the signal all the way back to the RK3399 chip itself, it is labeled there as B3. Experiment verifies that it is B3.
The driver is simple, you set a bit in the direction register to indicate the port is used for output, then you set and clear the same bit in the data register and it all works.
I use a delay routine that counts a huge number of times to achieve something that approximates a 1 second delay.
Tom's electronics pages / firstname.lastname@example.org