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/percpu.h

@@ -16,8 +16,6 @@
 #ifndef __ASM_PERCPU_H
 #define __ASM_PERCPU_H
 
-#ifdef CONFIG_SMP
-
 static inline void set_my_cpu_offset(unsigned long off)
 {
 	asm volatile("msr tpidr_el1, %0" :: "r" (off) : "memory");
@@ -38,12 +36,6 @@ static inline unsigned long __my_cpu_offset(void)
 }
 #define __my_cpu_offset __my_cpu_offset()
 
-#else	/* !CONFIG_SMP */
-
-#define set_my_cpu_offset(x)	do { } while (0)
-
-#endif /* CONFIG_SMP */
-
 #define PERCPU_OP(op, asm_op)						\
 static inline unsigned long __percpu_##op(void *ptr,			\
 			unsigned long val, int size)			\