mm: add memory pressure notification support

Add mm_heap_free interface to pass remaining memory to memory pressure

Signed-off-by: yinshengkai <yinshengkai@xiaomi.com>

mm/umm_heap/umm_malloc.c

@@ -66,6 +66,10 @@ FAR void *malloc(size_t size)
     {
       set_errno(ENOMEM);
     }
+  else
+    {
+      mm_notify_pressure(mm_heapfree(USR_HEAP));
+    }
 
   return ret;
 #endif