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

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+.. SPDX-License-Identifier: GPL-2.0
+
+=========================================
+The device for Mediatek MT6370 RGB LED
+=========================================
+
+Description
+-----------
+
+The MT6370 integrates a four-channel RGB LED driver, designed to provide a
+variety of lighting effect for mobile device applications. The RGB LED devices
+includes a smart LED string controller and it can drive 3 channels of LEDs with
+a sink current up to 24mA and a CHG_VIN power good indicator LED with sink
+current up to 6mA. It provides three operation modes for RGB LEDs:
+PWM Dimming mode, breath pattern mode, and constant current mode. The device
+can increase or decrease the brightness of the RGB LED via an I2C interface.
+
+The breath pattern for a channel can be programmed using the "pattern" trigger,
+using the hw_pattern attribute.
+
+/sys/class/leds/<led>/hw_pattern
+--------------------------------
+
+Specify a hardware breath pattern for a MT6370 RGB LED.
+
+The breath pattern is a series of timing pairs, with the hold-time expressed in
+milliseconds. And the brightness is controlled by
+'/sys/class/leds/<led>/brightness'. The pattern doesn't include the brightness
+setting. Hardware pattern only controls the timing for each pattern stage
+depending on the current brightness setting.
+
+Pattern diagram::
+
+     "0 Tr1 0 Tr2 0 Tf1 0 Tf2 0 Ton 0 Toff" --> '0' for dummy brightness code
+
+      ^
+      |           ============
+      |          /            \                                /
+Icurr |         /              \                              /
+      |        /                \                            /
+      |       /                  \                          /   .....repeat
+      |      /                    \                        /
+      |   ---                      ---                  ---
+      |---                            ---            ---
+      +----------------------------------============------------> Time
+       < Tr1><Tr2><   Ton    ><Tf1><Tf2 ><  Toff    >< Tr1><Tr2>
+
+Timing description::
+
+Tr1:    First rising time for duty 0 to 30%.
+Tr2:    Second rising time for duty 31% to 100%.
+Ton:    On time for duty 100%.
+Tf1:    First falling time for duty 100% to 31%.
+Tf2:    Second falling time for duty 30% to 0%.
+Toff:   Off time for duty 0%.
+
+Tr1/Tr2/Tf1/Tf2/Ton: 125ms to 3125ms, 200ms per step.
+Toff: 250ms to 6250ms, 400ms per step.
+
+Pattern example::
+
+       "0 125 0 125 0 125 0 125 0 625 0 1050"
+
+This Will configure Tr1/Tr2/Tf1/Tf2 to 125m, Ton to 625ms, and Toff to 1050ms.