powerpc/fixmap: Fix FIX_EARLY_DEBUG_BASE when page size is 256k

FIX_EARLY_DEBUG_BASE reserves a 128k area for debuging.

When page size is 256k, the calculation results in a 0 number of
pages, leading to the following failure:

  CC      arch/powerpc/kernel/asm-offsets.s
In file included from ./arch/powerpc/include/asm/nohash/32/pgtable.h:77:0,
                 from ./arch/powerpc/include/asm/nohash/pgtable.h:8,
                 from ./arch/powerpc/include/asm/pgtable.h:20,
                 from ./include/linux/pgtable.h:6,
                 from ./arch/powerpc/include/asm/kup.h:42,
                 from ./arch/powerpc/include/asm/uaccess.h:9,
                 from ./include/linux/uaccess.h:11,
                 from ./include/linux/crypto.h:21,
                 from ./include/crypto/hash.h:11,
                 from ./include/linux/uio.h:10,
                 from ./include/linux/socket.h:8,
                 from ./include/linux/compat.h:15,
                 from arch/powerpc/kernel/asm-offsets.c:14:
./arch/powerpc/include/asm/fixmap.h:75:2: error: overflow in enumeration values
  __end_of_permanent_fixed_addresses,
  ^
make[2]: *** [arch/powerpc/kernel/asm-offsets.s] Error 1

Ensure the debug area is at least one page.

Fixes: b8e8efaa ("powerpc: reserve fixmap entries for early debug")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/ca8c9f8249f523b1fab873e67b81b11989d46553.1592207216.git.christophe.leroy@csgroup.eu