AP_HAL: added dma allocation routines to hal.util

libraries/AP_HAL/Util.h

@@ -111,6 +111,10 @@ public:
     // create a new semaphore
     virtual Semaphore *new_semaphore(void) { return nullptr; }
 
+    // allocate and free DMA-capable memory if possible. Otherwise return normal memory
+    virtual void *dma_allocate(size_t size) { return malloc(size); }
+    virtual void dma_free(void *ptr, size_t size) { return free(ptr); }
+    
 protected:
     // we start soft_armed false, so that actuators don't send any
     // values until the vehicle code has fully started