docs/releases/4.21: Add toolchain updates

Change-Id: Ibd7c23a8c7c30ff19e0564d01489fecb3f34dadc
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/75414
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Documentation/releases/coreboot-4.21-relnotes.md

@@ -20,6 +20,11 @@ Significant or interesting changes
 * This section should have full descriptions and can or should have
   a link to the referenced commits.
 
+### Toolchain updates
+
+* Upgrade IASL from 20221020 to 20230331
+* Upgrade LLVM from 15.0.7 to 16.0.4
+
 Additional coreboot changes
 ---------------------------