sh: Definitions for 3-level page table layout

If using 64-bit PTEs and 4K pages then each page table has 512 entries
(as opposed to 1024 entries with 32-bit PTEs). Unlike MIPS, SH follows
the convention that all structures in the page table (pgd_t, pmd_t,
pgprot_t, etc) must be the same size. Therefore, 64-bit PTEs require
64-bit PGD entries, etc. Using 2-levels of page tables and 64-bit PTEs
it is only possible to map 1GB of virtual address space.

In order to map all 4GB of virtual address space we need to adopt a
3-level page table layout. This actually works out better for
CONFIG_SUPERH32 because we only waste 2 PGD entries on the P1 and P2
areas (which are untranslated) instead of 256.

Signed-off-by: Matt Fleming <matt@console-pimps.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>