sh: Establish a SuperHyway<->PCIe window mapping on SH7786 PCIe.

This bumps up the low address to match the physical memory windows for
SHway<->PCIe transfers. The previous implementation was banking on a 1:1
virt<->phys SHway mapping, which doesn't apply here.

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