Enable 32 bit GPIO input

[drtrigon]

Based on YosysHQ/picorv32#92 I enhanced the design to 32 output pins as shown here (renamed leds[7:0] to outp[31:0]):

How to add a control register that allows to switch single bits from output to input? How to add input pin functionality?

[drtrigon]

See YosysHQ/picorv32#96 (comment).

May be you can integrate this example into the repo as well in order to be helpful to others too?

However one question remains; Can you explain to me how to use the tri-state block? I placed and wired it - verification was ok but no bitstream could be generated, the result was an error.