Fix some correctness issues in the lowelevel kernel bringup code.

Fix some correctness issues in the lowelevel kernel bringup code.

- Make sure we install a dummy page table in TTBR0_EL1 before we change
  the size of the VA space in TCR_EL1.

- Flush the TLB after updating TCR_EL1.

- Flush TLB after installing the real kernel page table in TTBR1_EL1.

- Add some barriers around TLB flushes to make it consistent with
  other places where we do TLB flushes.

ok drahn@, patrick@