pci: sh7751: map PCI memory space into SDRAM

pci: sh7751: map PCI memory space into SDRAM

For ease of use and accounting a condition that on SH4
pci_phys_to_bus() and pci_bus_to_phys() are one in one mappings due to
unimplemented __iomem() conversion, this change fixes access to SDRAM
memory by PCI devices.

This change also generalizes PCI system memory configuration, which is
taken from board specific defines rather than hardcoded in the PCI
host driver.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>