doc: dts: Include #cells in legacy syntax section

Give an example for an interrupt controller, where 'interrupt-cells'
should be used instead.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>

doc/guides/dts/index.rst

@@ -485,6 +485,12 @@ The binding below shows various legacy syntax.
                type: int
                category: required
 
+   # Assume this is a binding for an interrupt controller
+   "#cells":
+       - irq
+       - priority
+       - flags
+
 This should now be written like this:
 
 .. code-block:: yaml
@@ -512,6 +518,11 @@ This should now be written like this:
                type: int
                required: true
 
+   interrupt-cells:
+       - irq
+       - priority
+       - cells
+
 The legacy syntax is still supported for backwards compatibility, but generates
 deprecation warnings. Support will be dropped in the Zephyr 2.3 release.