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>