x86: Add initial memory barrier macros

These are available on other architectures, so add them on x86.

Signed-off-by: Simon Glass <sjg@chromium.org>

arch/x86/include/asm/io.h

@@ -234,4 +234,12 @@ static inline phys_addr_t virt_to_phys(void * vaddr)
 	return (phys_addr_t)(vaddr);
 }
 
+/*
+ * TODO: The kernel offers some more advanced versions of barriers, it might
+ * have some advantages to use them instead of the simple one here.
+ */
+#define dmb()		__asm__ __volatile__ ("" : : : "memory")
+#define __iormb()	dmb()
+#define __iowmb()	dmb()
+
 #endif