sh: Acquire some more page flags for SH-5.

We need some more page flags to hook up _PAGE_WIRED (and eventually
other things). So use the unused PTE bits above the PPN field as no
implementations use these for anything currently.

Now that we have _PAGE_WIRED let's provide the SH-5 functions for wiring
up TLB entries.

Signed-off-by: Matt Fleming <matt@console-pimps.org>