Make use of bi-directional joystick ports (R4/R5)

[sy2002]

Newer MEGA65 boards support bi-directional joystick ports.

Info/quotes from a Skype conversation from the MEGA65 team why this feature was added:

• "I think this was to support joypads that use a biderectional protocol... was it the upcoming protopad?"
• "Its about supporting some of the multi-button joysticks that are on, or are going to be on the market. Possibly some numeric key pad things that connected to the joystick port, also."

For now - as long as we don't have a test case/use case, we cannot yet implement anything to the C64 core.