ppc: Add mmu_model defines for arch 2.03 and 2.07

ppc: Add mmu_model defines for arch 2.03 and 2.07

This removes unused POWERPC_MMU_2_06a/POWERPC_MMU_2_06d.

This replaces POWERPC_MMU_64B with POWERPC_MMU_2_03 for POWER5+ to be
more explicit about the version of the PowerISA supported.

This defines POWERPC_MMU_2_07 and uses it for the POWER8 CPU family.
This will not have an immediate effect now but it will in the following
patch.

This should cause no behavioural change.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
[aik: rebased, changed commit log]
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>