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>

Author: Marek Vasut

Documentation/devicetree/bindings/leds/common.yaml

@@ -79,24 +79,27 @@ properties:
       the LED.
     $ref: /schemas/types.yaml#/definitions/string
 
-    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
-        # Documentation/devicetree/bindings/leds/leds-gpio.yaml)
-      - default-on
-        # LED "double" flashes at a load average based rate
-      - heartbeat
-        # LED indicates disk activity
-      - disk-activity
-        # LED indicates IDE disk activity (deprecated), in new implementations
-        # use "disk-activity"
-      - ide-disk
-        # LED flashes at a fixed, configurable rate
-      - timer
-        # LED alters the brightness for the specified duration with one software
-        # timer (requires "led-pattern" property)
-      - pattern
+    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
+            # Documentation/devicetree/bindings/leds/leds-gpio.yaml)
+          - default-on
+            # LED "double" flashes at a load average based rate
+          - heartbeat
+            # LED indicates disk activity
+          - disk-activity
+            # LED indicates IDE disk activity (deprecated), in new implementations
+            # use "disk-activity"
+          - ide-disk
+            # LED flashes at a fixed, configurable rate
+          - timer
+            # 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: |