doc: release-notes-4-2: new DTS properties for the xSPI of STM32 devices

Change to apply on the DTS of STM32 soc and boards with xSPI nodes

Signed-off-by: Francois Ramu <francois.ramu@st.com>

Author: FRASTM

doc/releases/migration-guide-4.2.rst

@@ -360,6 +360,19 @@ SPI
 * Renamed the device tree property ``port_sel`` to ``port-sel``.
 * Renamed the device tree property ``chip_select`` to ``chip-select``.
 
+xSPI
+====
+
+* On STM32 devices, external memories device tree descriptions for size and address are now split
+  in two separate properties to comply with specification recommendations.
+
+  For instance, following external flash description ``reg = <0x70000000 DT_SIZE_M(64)>; /* 512 Mbits /``
+  is changed to ``reg = <0>;`` ``size = <DT_SIZE_M(512)>; / 512 Mbits */``.
+
+  Note that the property gives the actual size of the memory device in bits.
+  Previous mapping address information is now described in xspi node at SoC dtsi level.
+
+
 Other subsystems
 ****************