sh: Build PMB entry links for existing contiguous multi-page mappings.

This plugs in entry sizing support for existing mappings and then builds
on top of that for linking together entries that are mapping contiguous
areas. This will ultimately permit us to coalesce mappings and promote
head pages while reclaiming PMB slots for dynamic remapping.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>