PCI: keystone: Use platform_get_resource_byname() to get memory resources

Use platform_get_resource_byname() instead of platform_get_resource()
which uses an index to get memory resources. While at that get the memory
resource defined specifically for configuration space instead of
deriving the configuration space address from dbics address space.

Since the pci-keystone driver has never worked in the mainline kernel,
DT backward compatibility is not an issue.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>