dt-bindings: leds: Document mmc trigger

The mmc subsystem supports triggering leds on card activity, document
the trigger value here. The value is a pattern in this case.
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: devicetree@vger.kernel.org
To: linux-leds@vger.kernel.org
Signed-off-by: Pavel Machek <pavel@ucw.cz>
Reviewed-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/leds/common.yaml

@@ -79,7 +79,8 @@ properties:
       the LED.
     $ref: /schemas/types.yaml#/definitions/string
 
-    enum:
+    oneOf:
+      - enum:
             # LED will act as a back-light, controlled by the framebuffer system
           - backlight
             # LED will turn on (but for leds-gpio see "default-state" property in
@@ -97,6 +98,8 @@ properties:
             # LED alters the brightness for the specified duration with one software
             # timer (requires "led-pattern" property)
           - pattern
+        # LED is triggered by SD/MMC activity
+      - pattern: "^mmc[0-9]+$"
 
   led-pattern:
     description: |