[media] of: v4l: Document link-frequencies property in video-interfaces.txt

link-frequencies is a 64-bit unsigned integer array of allowed link
frequencies.
Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

Documentation/devicetree/bindings/media/video-interfaces.txt

@@ -103,6 +103,9 @@ Optional endpoint properties
   array contains only one entry.
 - clock-noncontinuous: a boolean property to allow MIPI CSI-2 non-continuous
   clock mode.
+- link-frequencies: Allowed data bus frequencies. For MIPI CSI-2, for
+  instance, this is the actual frequency of the bus, not bits per clock per
+  lane value. An array of 64-bit unsigned integers.
 
 
 Example