dm: pci: add option to map virtual system memory base address

On MIPS the DRAM start address respectively CONFIG_SYS_SDRAM_BASE
is still used as a virtual, CPU-mapped address instead of being used
as physical address. Converting all MIPS boards and generic MIPS code
to fix that is not trivial. Due to the approaching deadline for
PCI DM conversion, this workaround is required for MIPS boards with
PCI support until the CONFIG_SYS_SDRAM_BASE issue could be solved.

Add a compile-time option to let the PCI uclass core optionally map
the DRAM address to a physical address when adding the PCI region
of type PCI_REGION_SYS_MEMORY.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>

Reviewed-by: Stefan Roese <sr@denx.de>