Add a meaningful alias for `rKEY0`

[Rangi42]

hardware.inc version 3.0 (PR #4) added the rSPD alias for rKEY1 ($FF4D). However, the newly-defined rKEY0 ($FF4C) doesn't have a meaningful name.

Nintendo's patent US6322447B1 calls it a "CPU mode" register in Figure 11 (and KEY1/SPD is called "CPU operation speed").

A few proposals:

• rMODE
• rSYS
• rCPU

[ISSOtm]

rCOMPAT?

[Rangi42]

"DMG compatibility" mode is just one of the four values this can take; and Pan Docs calls the register itself "CPU mode select".

[ISSOtm]

Then rSYS has my vote.

[Rangi42]

"The SYStem is CGB", sure.