doc: release-notes-4.0: add release notes for SD subsystem

No significant changes have been made to the SD subsystem since 3.7. Add
a note documenting this for the 4.0 release.

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

Author: danieldegrasse

doc/releases/release-notes-4.0.rst

@@ -1088,6 +1088,8 @@ Libraries / Subsystems
 
 * SD
 
+  * No significant changes in this release
+
 * Settings
 
   * Settings has been extended to allow prioritizing the commit handlers using