doc: release notes: a note for changing the default FP mode in ARM

Adding a note in the v2.5.0 release notes to reflect
that we have changed the default FP services mode from
unshared to shared, for ARM Cortex-M.

Signed-off-by: Ioannis Glaropoulos <[email protected]>

Author: ioannisg

doc/releases/release-notes-2.5.rst

@@ -154,6 +154,8 @@ Architectures
     * Introduced the functionality for chain-loadable Zephyr
       fimrmware images to force the initialization of internal
       architecture state during early system boot (Cortex-M).
+    * Changed the default Floating Point Services mode to
+      Shared FP registers mode.
 
   * AARCH64