[Keyboard] Change Corne RGB Matrix split handling (#7117)

[Keyboard] Change Corne RGB Matrix split handling (#7117)

* [Keyboard] Change Corne RGB Matrix split handling

This uses the "is_master" detection to set the led matrix, rather than a define at compile time.

This means that the same hex can be used for both halves, not just one or the other.  The caveat is that this costs ~240 bytes to do.
However, I feel that this is a good trade-off, not just lazy.

* Update documentation for RGB Matrix on the Corne