arm64: force CONFIG_SMP=y and remove redundant #ifdefs

Nobody seems to be producing !SMP systems anymore, so this is just
becoming a source of kernel bugs, particularly if people want to use
coherent DMA with non-shared pages.

This patch forces CONFIG_SMP=y for arm64, removing a modest amount of
code in the process.

Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/assembler.h

@@ -91,9 +91,7 @@
  * SMP data memory barrier
  */
 	.macro	smp_dmb, opt
-#ifdef CONFIG_SMP
 	dmb	\opt
-#endif
 	.endm
 
 #define USER(l, x...)				\