ARM: 8101/1: mach-iop13xx: fix possible build failure

ARM: 8101/1: mach-iop13xx: fix possible build failure

After applying patch:
	"ARM: 8078/1: get rid of hardcoded assumptions about kernel stack size"

following build failure happens on iop13xx platform:

   In file included from include/linux/srcu.h:33:0,
                    from include/linux/notifier.h:15,
                    from include/linux/reboot.h:5,
                    from arch/arm/mach-iop13xx/include/mach/iop13xx.h:6,
                    from arch/arm/mach-iop13xx/include/mach/hardware.h:14,
                    from arch/arm/mach-iop13xx/include/mach/memory.h:4,
                    from arch/arm/include/asm/memory.h:24,
                    from arch/arm/include/asm/page.h:163,
                    from arch/arm/include/asm/thread_info.h:17,
                    from include/linux/thread_info.h:54,
                    from include/asm-generic/preempt.h:4,
                    from arch/arm/include/generated/asm/preempt.h:1,
                    from include/linux/preempt.h:18,
                    from include/linux/spinlock.h:50,
                    from include/linux/seqlock.h:35,
                    from include/linux/time.h:5,
                    from include/uapi/linux/timex.h:56,
                    from include/linux/timex.h:56,
                    from include/linux/sched.h:19,
                    from arch/arm/kernel/asm-offsets.c:13:
   include/linux/rcupdate.h: In function '__rcu_read_lock':
>> include/linux/rcupdate.h:220:2: error: implicit declaration of function 'preempt_disable' [-Werror=implicit-function-declaration]
     preempt_disable();

The problem here is recursive header inclusion which could be avoided by
removing linux/reboot.h from mach/iop13xxx.h.
linux/reboot.h in include/mach/iop13xx.h is needed only for enum reboot_mode,
so header it could be replaced with a enum declaration.

Whatever patch "ARM: 8078/1: get rid of hardcoded assumptions about kernel stack size"
does, I think it's good to avoid unnecessary header inclusion here in any case.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrey Ryabinin <a.ryabinin@samsung.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>