doc: Add a new migration guide entry to counter

Rename the primary_source, secondary_source,
filter_count and filter_period property to
primary-source,  secondary-source, filter-count
and filter-period in the devicetree and bindings
of the counter subsystem.

Signed-off-by: James Roy <rruuaanng@outlook.com>

doc/releases/migration-guide-4.1.rst

@@ -115,6 +115,14 @@ ADC
 
 * Renamed the ``compatible`` from ``nxp,kinetis-adc12`` to :dtcompatible:`nxp,adc12`.
 
+Counter
+=======
+
+* Renamed the devicetree property ``primary_source`` to ``primary-source``.
+* Renamed the devicetree property ``secondary_source`` to ``secondary-source``.
+* Renamed the devicetree property ``filter_count`` to ``filter-count``.
+* Renamed the devicetree property ``filter_period`` to ``filter-period``.
+
 Controller Area Network (CAN)
 =============================