doc: Migration guide v4.1: STM32 addition for the MCO

STM32 configuration for the MCO clock source and prescaler is done
by the DTS and no more by Kconfig (removed).

Signed-off-by: Francois Ramu <[email protected]>

Author: FRASTM

doc/releases/migration-guide-4.1.rst

@@ -24,6 +24,17 @@ Kernel
 Boards
 ******
 
+Devicetree
+**********
+
+STM32
+=====
+
+* MCO clock source and prescaler are now exclusively configured by the DTS
+  as it was introduced earlier.
+  The Kconfig method for configuration is now removed.
+
+
 Modules
 *******