PCI: designware: Look for configuration space in 'reg', not 'ranges'

The configuration address space has so far been specified in *ranges*,
however it should be specified in *reg* making it a platform MEM resource.
Hence used 'platform_get_resource_*' API to get configuration address space
in the designware driver.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Mohit Kumar <mohit.kumar@st.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Cc: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Arnd Bergmann <arnd@arndb.de>