doc: release-notes/3.2: about new API on MM drivers

This adds the bits about new API in the memory management drivers.

Signed-off-by: Daniel Leung <[email protected]>

Author: dcpleung

doc/releases/release-notes-3.2.rst

@@ -72,6 +72,12 @@ Changes in this release
     allow distinguishing between an out of range bitrate/sample point, an unsupported bitrate, and a
     resulting sample point outside the guard limit.
 
+* Memory Management Drivers
+
+  * Added :c:func:`sys_mm_drv_update_page_flags` and
+    :c:func:`sys_mm_drv_update_region_flags` to update flags associated
+    with memory pages and regions.
+
 Removed APIs in this release
 ============================