arm64: Implement cpu_relax as yield

ARM64 has the yield nop hint which has the intended semantics of
cpu_relax. Implement.

The immediate application is ARM CPU emulators. An emulator can take
advantage of the yield hint to de-prioritise an emulated CPU in favor
of other emulation tasks. QEMU A64 SMP emulation has yield awareness,
and sees a significant boot time performance increase with this change.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/processor.h

@@ -127,7 +127,11 @@ extern void release_thread(struct task_struct *);
 
 unsigned long get_wchan(struct task_struct *p);
 
-#define cpu_relax()			barrier()
+static inline void cpu_relax(void)
+{
+	asm volatile("yield" ::: "memory");
+}
+
 #define cpu_relax_lowlatency()                cpu_relax()
 
 /* Thread switching */