Clean the breathe-apidoc files with make clean

Signed-off-by: David Horstmann <david.horstmann@arm.com>

docs/Makefile

diff --git a/docs/Makefile b/docs/Makefile
index 6a7599e..47510f9 100644
--- a/docs/Makefile
+++ b/docs/Makefile
@@ -18,6 +18,8 @@
 clean:
 	@# Clean the apidoc
 	$(MAKE) -C .. apidoc_clean
+	@# Clean the breathe-apidoc generated files
+	rm -rf ./api
 	@# Clean the sphinx docs
 	@$(SPHINXBUILD) -M clean "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)