Doc: Release Notes: Add util_memeq and util_eq

Add the two new function `util_memeq` and `util_eq` recently added in
`sys/util.h` to the release notes for 4.2.

Signed-off-by: Théo Battrel <theo.battrel@nordicsemi.no>

Author: theob-pro

doc/releases/release-notes-4.2.rst

@@ -167,6 +167,11 @@ New APIs and options
 
   * :c:func:`counter_reset`
 
+* Sys
+
+  * :c:func:`util_eq`
+  * :c:func:`util_memeq`
+
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