cortex-m: update changelog after rust-embedded#539

Author: newAM

cortex-m/CHANGELOG.md

@@ -7,8 +7,14 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 
+## Changed
+
 - Bumped MSRV to 1.61 for compatibility with syn versions >=2.0.68 and to match cortex-m-rt crate
 
+### Added
+
+- Added `set_sevonpend` and `clear_sevonpend` (#539).
+
 ## [v0.7.7] - 2023-01-03
 
 - Add missing documentation for `critical-section-single-core` feature added