doc: Update SoftDevice Controller release notes

Added section about v2.1.2, and note about Errata 158.

Signed-off-by: Martin Tverdal <martin.tverdal@nordicsemi.no>

Author: martintv

softdevice_controller/CHANGELOG.rst

@@ -9,6 +9,16 @@ Changelog
 
 All the notable changes to this project are documented in this file.
 
+nRF Connect SDK v2.1.2
+**********************
+
+All the notable changes included in the |NCS| v2.1.2 release are documented in this section.
+
+Changes
+=======
+
+* On nRF53, the fix for Errata 158 is now applied.
+
 nRF Connect SDK v2.1.0
 **********************