northbridge/amd/{gx2,lx}: Qualify pointer with `volatile`

There is no guarantee reading a dereferenced null pointer will not be
optimised away. Qualify the integer storage type with volatile. Clang
enforces this explicitness.

Change-Id: I31524141d70632cade0490c820936a3a8b570346
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/6148
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>

src/northbridge/amd/gx2/northbridgeinit.c

@@ -660,7 +660,7 @@ void northbridge_init_early(void)
 
 	/* Now that the descriptor to memory is set up. */
 	/* The memory controller needs one read to synch its lines before it can be used. */
-	i = *(int *) 0;
+	i = *(volatile int *) 0;
 
 	GeodeLinkPriority();
 

src/northbridge/amd/lx/northbridgeinit.c

@@ -742,7 +742,7 @@ void northbridge_init_early(void)
 
 	/*  Now that the descriptor to memory is set up. */
 	/*  The memory controller needs one read to synch its lines before it can be used. */
-	i = *(int *)0;
+	i = *(volatile int *)0;
 
 	GeodeLinkPriority();