Add constants for SGB command `MLT_REQ`'s use of `rJOYP`

There are [many SGB commands](https://gbdev.io/pandocs/SGB_Command_Summary.html), but `MLT_REQ` stands out as the "[tried and true](https://github.com/gbdev/pandocs/issues/442#issuecomment-3181838936)" one for detecting SGB in the first place; and in some games ([e.g. Pokémon Red/Green](https://github.com/pret/pokered/commit/1eb6f61d62133922ac15c61792be8ea901d6baba)) it's the only you see apart from the necessary border transfers.

`MLT_REQ` uses bits 0 and 1 of `rJOYP` (`rP1`, `$FF00`) for [multiplayer control](https://gbdev.io/pandocs/SGB_Command_Multiplayer.html#sgb-command-11--mlt_req).