arm64: remove dead code

Commit 68234df4ea ("arm64: kill flush_cache_all()") removed
soft_reset() from the kernel. This was the only caller of
setup_mm_for_reboot(), so remove that also.

Signed-off-by: Mark Salter <msalter@redhat.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/mmu.h

@@ -28,7 +28,6 @@ typedef struct {
 #define ASID(mm)	((mm)->context.id & 0xffff)
 
 extern void paging_init(void);
-extern void setup_mm_for_reboot(void);
 extern void __iomem *early_io_map(phys_addr_t phys, unsigned long virt);
 extern void init_mem_pgprot(void);
 extern void create_pgd_mapping(struct mm_struct *mm, phys_addr_t phys,