powerpc/book-3e: Introduce concept of Book-3e MMU

The Power ISA 2.06 spec introduces a standard MMU programming model that
is based on the Freescale Book-E MMU programing model.  The Freescale
version is pretty backwards compatiable with the ISA 2.06 definition so
we are starting to refactor some of the Freescale code so it can be
easily shared.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>