doc: release-notes: Added note on TF-M 1.6.0

Add a note on the update to TF-M 1.6.0 and MbedTLS 3.1.

Signed-off-by: Kevin Townsend <[email protected]>

Author: microbuilder

doc/releases/release-notes-3.1.rst

@@ -405,6 +405,8 @@ MCUboot
 Trusted Firmware-m
 ******************
 
+* Updated to TF-M 1.6.0
+
 Documentation
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