powerpc: Add missing NULL terminator to avoid boot panic on PPC40x

The missing NULL terminator can cause a panic on
PPC405 boards during boot:

  Linux/PowerPC load: console=ttyS0,115200 root=/dev/mtdblock1 rootfstype=squashfs,jffs2 noinitrd init=/etc/preinit
  Finalizing device tree... flat tree at 0x6a5160
  bootconsole [udbg0] enabled
  Page fault in user mode with in_atomic() = 1 mm = (null)
  NIP = c0275f50  MSR = fffffffe
  Oops: Weird page fault, sig: 11 [#1]
  PowerPC 40x Platform
  Modules linked in:
  NIP: c0275f50 LR: c0275f60 CTR: c0280000
  REGS: c0275eb0 TRAP: 636f7265   Not tainted  (3.7.1)
  MSR: fffffffe <VEC,VSX,EE,PR,FP,ME,SE,BE,IR,DR,PMM,RI> CR: c06a6190  XER: 00000001
  TASK = c02662a8[0] 'swapper' THREAD: c0274000
  GPR00: c0275ec0 c000c658 c027c4bf 00000000 c0275ee0 c000a0ec c020a1a8 c020a1f0
  GPR08: c020f631 c020f404 c025f078 c025f080 c0275f10
   Call Trace:
   ---[ end trace 31fd0ba7d8756001 ]---

  Kernel panic - not syncing: Attempted to kill the idle task!

The panic happens since commit 9597abe0
(sections: fix section conflicts in arch/powerpc), however the root
cause of this is that the NULL terminator were not added in commit
a4f740cf (of/flattree: Add of_flat_dt_match()
helper function).

Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: <stable@vger.kernel.org>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>