Add clangd .cache folder to .gitignore

Clangd is a commonly used language server and it generates a cache
folder in the root of the project that we can ignore.

Signed-off-by: Mudit Sharma <mudit.sharma@arm.com>
Change-Id: I60721a060d5548b940279859de01e81dd73a537f

Author: muditsharmaa-arm

.gitignore

@@ -27,3 +27,6 @@ venv
 .venv
 
 trusted_firmware_m.egg-info/
+
+# clangd
+.cache/