doc: releases: Add ElemRV to Release Notes

Add a note about initial support for ElemRV-N.

Signed-off-by: Daniel Schultz <dnltz@aesc-silicon.de>

Author: dnltz

doc/releases/release-notes-4.2.rst

@@ -257,6 +257,10 @@ New Boards
 
    * :zephyr:board:`versalnet_rpu` (``versalnet_rpu``)
 
+* Aesc Silicon
+
+   * :zephyr:board:`elemrv` (``elemrv``)
+
 * aithinker
 
    * :zephyr:board:`ai_wb2_12f` (``ai_wb2_12f``)