dt-bindings: leds: Document commonly used LED triggers

Document the commonly used LED triggers by the SoCs. Not all triggers
are documented as some of them are very application specific. Most of the
triggers documented here are currently used in devicetrees of many SoCs.

While at it, add missing comments and also place the comment above the
triggers (hci, mmc, wlan) to match the rest of the binding.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230303122925.6610-1-manivannan.sadhasivam@linaro.org