sh: Fix up multi-resource mapping for SH7786 PCIe.

This reworks some of the SH7786 PCIe initialization code to dynamically
setup and size the various resource windows, as opposed to the original
code that simply wired in a couple of them statically.

At the same time, we tidy up the initialization code a bit, kill off some
read-only register twiddling that was gleaned from the bus analyzer, and
also propagate the physical slot/channel mapping.

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