ppc: Add/Re-introduce MMU model definitions needed by PR KVM

Commit aa4bb5875231 (ppc: Add mmu_model defines for arch 2.03 and 2.07)
removed the mmu_model definition POWERPC_MMU_2_06a which is needed by
PR KVM. Reintroduce it and also add POWERPC_MMU_2_07a.

This fixes QEMU crash (qemu: fatal: Unknown MMU model) during booting
of PR KVM guest.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
