Adds support for changing the laptop keyboard LED mode and colour.

The modes are visible effects such as static, rainbow, pulsing,
colour cycles.

These sysfs attributes are added to asus::kbd_backlight:
- kbd_rgb_mode
- kbd_rgb_mode_index
Signed-off-by: Luke D. Jones <luke@ljones.dev>
Link: https://lore.kernel.org/r/20220825232251.345893-2-luke@ljones.devReviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>