platform/x86: system76: Handle new KBLED ACPI methods

platform/x86: system76: Handle new KBLED ACPI methods

System76 EC since system76/ec@9ac513128ad9 detects if the keyboard is
white or RGB backlit via `RGBKB-DET#` at run-time instead of being set
at compile-time. As part of this, the brightness of white-only backlit
keyboards was also changed to behave more like the RGB-backlit
keyboards: a value between 0 and 255 instead of a firmware-defined
level.

The EC ACPI methods in coreboot have been updated for this new
functionality only, removing the old behavior.

This should preserve behavior as we roll out new firmware with these
changes included and users update to it.

Link: https://github.com/system76/ec/pull/357
Link: https://review.coreboot.org/c/coreboot/+/76152
Signed-off-by: Tim Crawford <tcrawford@system76.com>
Link: https://lore.kernel.org/r/20230719181324.47035-1-tcrawford@system76.com
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>