sh: pci: Rewrite SH7751 PCI support to follow SH7780.

This follows the similar sort of scheme that the refactored SH7780 code
uses, using a 64MB CS3 mapping to handle the window0 case, and simply
discarding window1. This vastly simplifies the code, and allows most of
the board-specific setup to go die.

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