powerpc/mm: Fix compile error of pgtable-ppc64.h

It seems that forward declaration couldn't work well with typedef, use
struct spinlock directly to avoiding following build errors:

In file included from include/linux/spinlock.h:81,
                 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:17,
                 from arch/powerpc/kernel/asm-offsets.c:17:
include/linux/spinlock_types.h:76: error: redefinition of typedef 'spinlock_t'
/root/linux-next/arch/powerpc/include/asm/pgtable-ppc64.h:563: note: previous declaration of 'spinlock_t' was here

Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>