csky: Use generic asid algorithm to implement switch_mm

Use linux generic asid/vmid algorithm to implement csky
switch_mm function. The algorithm is from arm and it could
work with SMP system. It'll help reduce tlb flush for
switch_mm in task/vm switch.

Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Cc: Arnd Bergmann <arnd@arndb.de>

arch/csky/include/asm/mmu.h

@@ -5,6 +5,7 @@
 #define __ASM_CSKY_MMU_H
 
 typedef struct {
+	atomic64_t	asid;
 	void *vdso;
 } mm_context_t;