powerpc/8xx: Remove _PAGE_USER and handle user access at PMD level

As Linux kernel separates KERNEL and USER address spaces, there is
therefore no need to flag USER access at page level.

Today, the 8xx TLB handlers already handle user access in the L1 entry
through Access Protection Groups, it is then natural to move the user
access handling at PMD level once _PAGE_NA allows to handle PAGE_NONE
protection without _PAGE_USER

In the mean time, as we free up one bit in the PTE, we can use it to
include SPS (page size flag) in the PTE and avoid handling it at every
TLB miss hence removing special handling based on compiled page size.

For _PAGE_EXEC, we rework it to use PP PTE bits, avoiding the copy
of _PAGE_EXEC bit into the L1 entry. Unfortunatly we are not
able to put it at the correct location as it conflicts with
NA/RO/RW bits for data entries.

Upper bits of APG in L1 entry overlap with PMD base address. In
order to avoid having to filter that out, we set up all groups so that
upper bits can have any value.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>