doc: releases: migration guide entry

Added the removal of the 'amba-ahb-dbus-width' devicetree property
from the binding of the Xilinx GEM Ethernet controller driver to the
current migration guide.

Signed-off-by: Immo Birnbaum <[email protected]>

Author: ibirnbaum

doc/releases/migration-guide-4.3.rst

@@ -233,6 +233,11 @@ Ethernet
     * Replaced devicetree property ``tx-checksum-offload`` which enabled TX checksum offloading
       ``disable-tx-checksum-offload`` which now actively disables it.
 
+* The Xilinx GEM Ethernet driver (:dtcompatible:`xlnx,gem`) now obtains the AMBA AHB data bus
+  width matching the current target SoC (either Zynq-7000 or ZynqMP) from a design configuration
+  register at run-time, making the devicetree property ``amba-ahb-dbus-width`` obsolete, which
+  has therefore been removed.
+
 Power management
 ****************