Revert "arm64: no gmb() implementation yet"

CVE-2017-5753 (revert embargoed)
CVE-2017-5715 (revert embargoed)

This reverts commit 4e1b8db1.
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

arch/arm64/include/asm/barrier.h

@@ -31,7 +31,6 @@
 #define mb()		dsb(sy)
 #define rmb()		dsb(ld)
 #define wmb()		dsb(st)
-#define gmb()   	do { } while (0)
 
 #define dma_rmb()	dmb(oshld)
 #define dma_wmb()	dmb(oshst)