m68k: Switch to asm-generic/barrier.h

The generic nop() implementation is fine for m68k.
Signed-off-by: NGeert Uytterhoeven <geert@linux-m68k.org>
Acked-by: NGreg Ungerer <gerg@uclinux.org>

arch/m68k/include/asm/Kbuild

+generic-y += barrier.h
 generic-y += bitsperlong.h
 generic-y += clkdev.h
 generic-y += cputime.h

arch/m68k/include/asm/barrier.h

-#ifndef _M68K_BARRIER_H
-#define _M68K_BARRIER_H
-
-#define nop()		do { asm volatile ("nop"); barrier(); } while (0)
-
-#include <asm-generic/barrier.h>
-
-#endif /* _M68K_BARRIER_H */