nios2/mm: refactor swap PTE layout

nios2 disables swap for a good reason: it doesn't even provide sufficient
type bits as required by core MM.  However, swap entries are nowadays also
used for other purposes (migration entries, PTE markers, HWPoison, ...),
and accidential use could be problematic.

Let's properly use 5 bits for the swap type and document the layout.  Bits
26--31 should get ignored by hardware completely, so they can be used.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: David Hildenbrand <[email protected]>
Cc: Dinh Nguyen <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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