Missing `rJOYP` alias for `rP1`

[Rangi42]

Pan Docs lists $FF00 as "P1/JOYP", so I'd expect either to be usable.

[ISSOtm]

P1 sucks as a name, because that register is used to report any player's input (and its low 4 bits indicate the active player on SGB, too!). JOYP is better at least in that regard, so I'm seconding this.

[nitro2k01]

While there might be a risk for confusion, note that P doesn't stand for player, but port. Port 1 (P1) is a digital IO port with 2 outputs and 4 inputs. This is for example shown in the CPU block diagram in Nintendo's programming manual.

However, the alias rJOYP should definitely exist.