arm64: barriers: add dmb barrier

Commit 8adbf57f ("irqchip: gic: use dmb ishst instead of dsb when
raising a softirq") added an explicit dmb(...) call to the GIC driver.

This patch adds a simple dmb() macro to arm64, which expands to a DMB SY
instruction.
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/barrier.h

@@ -25,6 +25,7 @@
 #define wfi()		asm volatile("wfi" : : : "memory")
 
 #define isb()		asm volatile("isb" : : : "memory")
+#define dmb(opt)	asm volatile("dmb sy" : : : "memory")
 #define dsb(opt)	asm volatile("dsb sy" : : : "memory")
 
 #define mb()		dsb()