sh: Fix up dynamically created write-through PMB mappings.

Write-through PMB mappings still require the cache bit to be set, even if
they're to be flagged with a different cache policy and bufferability
bit. To reduce some of the confusion surrounding the flag encoding we
centralize the cache mask based on the system cache policy while we're at
it.

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