Make the minimal verilog actually minimal

[mithro]

Convert the file at https://github.com/im-tomu/fomu-workshop/blob/master/verilog-blink-minimal/blink.v to be more minimal.

• Remove the usage of the PLL, use a counter instead.
• Directly drive the pins rather than SB_RGBA_DRV

[xobs]

The blink example now uses a counter rather than the PLL, and does away with the touchpads and pmod stuff.

I feel uncomfortable removing SB_RGBA_DRV since directly driving the pins causes an overvoltage on the red LED on production boards, shortening its life.

[umarcor]

I believe the current examples are minimal enough. As discussed in the chat, the instantiation of SB_GB is redundant, because nextpnr will add it anyway. However, I believe keeping it is ok, because it takes 5 lines only, and it's good for didactic purposes. Regarding SB_RGBA_DRV, as @xobs said, it wouldn't be good practice to drive the LEDs directly. Therefore, I believe that this issue can be closed.