Add support for Master Compact's edge connector PB PINs

[DanCasterIt]

Hi, would it be possible to use the original Master user port PB7, PB6 and PB5 PINs instead of CB1 PB1 CB2 PB0?

If yes, that would allow using the card edege connector of the Master Compact instead of the joystick port allowing master compact users to use their mouse even when using MMFS

I also made a stardot post about it to better explain my idea: MMFS for the Master Compact's edge connector user port PINs

[hoglet67]

Response here: https://www.stardot.org.uk/forums/viewtopic.php?p=439180#p439180