ARM: 6939/1: fix missing 'cpu_relax()' declaration

ARM build fails with the following symptom:

  CC      arch/arm/kernel/asm-offsets.s
In file included from include/linux/seqlock.h:29,
                 from include/linux/time.h:8,
                 from include/linux/timex.h:56,
                 from include/linux/sched.h:57,
                 from arch/arm/kernel/asm-offsets.c:13:
include/linux/spinlock.h: In function 'spin_unlock_wait':
include/linux/spinlock.h:360: error: implicit declaration of function 'cpu_relax'
make[1]: *** [arch/arm/kernel/asm-offsets.s] Error 1

Fix it by including <asm/processor.h>.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>