docs: leds: Add MT6370 RGB LED pattern document

Document the MT6370 RGB LED pattern trigger.

This simply describe how the pattern works, each timing period, and the
pattern diagram for MT6370 RGB LED.

Signed-off-by: ChiYuan Huang <cy_huang@richtek.com>
Signed-off-by: ChiaEn Wu <chiaen_wu@richtek.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/38f1e863b0f099158a63fb6f95056a1cb30d80a0.1678430444.git.chiaen_wu@richtek.com