Use hf/call.h shared header.

Change-Id: I4f4ea20ad08d03ee46dfdd8fb76207e3b53e3b77
diff --git a/hf_call.S b/hf_call.S
new file mode 100644
index 0000000..07743ca
--- /dev/null
+++ b/hf_call.S
@@ -0,0 +1,6 @@
+.text
+
+.global hf_call
+hf_call:
+	hvc #0
+	ret