arm64: barriers: introduce nops and __nops macros for NOP sequences

NOP sequences tend to get used for padding out alternative sections
and uarch-specific pipeline flushes in errata workarounds.

This patch adds macros for generating these sequences as both inline
asm blocks, but also as strings suitable for embedding in other asm
blocks directly.
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/assembler.h

@@ -86,6 +86,15 @@
 	dmb	\opt
 	.endm
 
+/*
+ * NOP sequence
+ */
+	.macro	nops, num
+	.rept	\num
+	nop
+	.endr
+	.endm
+
 /*
  * Emit an entry into the exception table
  */

arch/arm64/include/asm/barrier.h

@@ -20,6 +20,9 @@
 
 #ifndef __ASSEMBLY__
 
+#define __nops(n)	".rept	" #n "\nnop\n.endr\n"
+#define nops(n)		asm volatile(__nops(n))
+
 #define sev()		asm volatile("sev" : : : "memory")
 #define wfe()		asm volatile("wfe" : : : "memory")
 #define wfi()		asm volatile("wfi" : : : "memory")