powerpc: Fixes for CONFIG_PTE_64BIT for SMP support

There are some minor issues with support 64-bit PTEs on a 32-bit processor
when dealing with SMP.

* We need to order the stores in set_pte_at to make sure the flag word
  is set second.
* Change pte_clear to use pte_update so only the flag word is cleared
* Added a WARN_ON to set_pte_at to ensure the pte isn't present for
  the 64-bit pte/SMP case (to ensure our assumption of this fact).

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Becky Bruce <becky.bruce@freescale.com>