sh: Ensure active regions have a backing PMB entry.

In the NUMA or memory hot-add case where system memory has been
partitioned up, we immediately run in to a situation where the existing
PMB entry doesn't cover the new range (primarily as a result of the entry
size being shrunk to match the node size early in the initialization). In
order to fix this up it's necessary to preload a PMB mapping for the new
range prior to activation in order to circumvent reset by MMU.

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