AP_RPM: add AP::rpm() call for singleton

libraries/AP_RPM/AP_RPM.cpp

@@ -182,3 +182,11 @@ bool AP_RPM::enabled(uint8_t instance) const
 // singleton instance
 AP_RPM *AP_RPM::_singleton;
 
+namespace AP {
+
+AP_RPM *rpm()
+{
+    return AP_RPM::get_singleton();
+}
+
+}

libraries/AP_RPM/AP_RPM.h

@@ -104,3 +104,7 @@ private:
     void detect_instance(uint8_t instance);
     void update_instance(uint8_t instance);
 };
+
+namespace AP {
+    AP_RPM *rpm();
+};