spec: fill missing changelog for unreleased 'binary: add counter index mask type'

Signed-off-by: Zhouqi Jiang <luojia@hust.edu.cn>

sbi-spec/CHANGELOG.md

@@ -9,6 +9,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 ### Added
 
 - base: add Coreboot and Oreboot to `impl_id` module
+- binary: add counter index mask type
 
 ### Modified