SPCI: Implement SPCI_VERSION.

Change-Id: I57b8c6f1fd12cbc37bd7f56b96d9fa0c241d3ba9
diff --git a/inc/hf/api.h b/inc/hf/api.h
index 1de07ec..259d59e 100644
--- a/inc/hf/api.h
+++ b/inc/hf/api.h
@@ -53,3 +53,4 @@
 int32_t api_spci_msg_recv(uint32_t attributes, struct vcpu *current,
 			  struct vcpu **next);
 int32_t api_spci_yield(struct vcpu *current, struct vcpu **next);
+int32_t api_spci_version(void);