arch/sh: pcie-sh7786: handle non-zero DMA offset

On SuperH, the base of the physical memory might be different from
zero. In this case, PCI address zero will map to a non-zero physical
address. In order to make sure that the DMA mapping API takes care of
this DMA offset, we must fill in the dev->dma_pfn_offset field for PCI
devices. This gets done in the pcibios_bus_add_device() hook, called
for each new PCI device detected.

The dma_pfn_offset global variable is re-calculated for every PCI
controller available on the platform, but that's not an issue because
its value will each time be exactly the same, as it only depends on
the memory start address and memory size.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Rich Felker <dalias@libc.org>