doc: releases: add migration guide entry

Add description of the changes made to the 4.4 migration guide.

Signed-off-by: Immo Birnbaum <mail@birnbaum.immo>

Author: ibirnbaum

doc/releases/migration-guide-4.4.rst

@@ -226,7 +226,6 @@ Controller Area Network (CAN)
   (:github:`103679`).
 
 Counter
-=======
 
 * Drivers that implement ``get_value_64`` API will now need to select
   :kconfig:option:`CONFIG_COUNTER_SUPPORTS_64BITS_TICKS` and applications will need
@@ -499,6 +498,15 @@ Ethernet
   reworked to be used as active low, you may have to set the pin as
   ``GPIO_ACTIVE_LOW`` in devicetree (:github:`100751`).
 
+* The Xilinx GEM Ethernet driver (:dtcompatible:`xlnx,gem`) has been switched over to use the
+  current MDIO and PHY facilities, splitting up the driver's implementation into separate
+  MDIO and Ethernet MAC drivers. The driver's custom PHY management code has been removed.
+  The types of Ethernet PHYs supported by the removed custom code, the Marvell Alaska GBit
+  PHY family and the TI TLK105/DP83822 100 MBit PHYs are both covered by the standard
+  (:dtcompatible:`ethernet-phy`) driver. The QEMU targets which emulate the Xilinx GEM have
+  been updated accordingly, as have been the device trees of the Zynq-7000 and ZynqMP /
+  UltraScale+ SoC families.
+
 File System
 ===========