s390x/mmu: DAT table definition overhaul

s390x/mmu: DAT table definition overhaul

Let's use consistent names for the region/section/page table entries and
for the macros to extract relevant parts from virtual address. Make them
match the definitions in the PoP - e.g., how the relevant bits are actually
called.

Introduce defines for all bits declared in the PoP. This will come in
handy in follow-up patches.

Add a note where additional information about s390x and the used
definitions can be found.

Acked-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>