PCI: designware: Add driver for prototyping kits based on ARC SDP

Add a reference platform driver for PCI RC IP Protoyping Kits based on the
ARC SDP.

[bhelgaas: changelog, split patch up, MAINTAINERS update]
Signed-off-by: Joao Pinto <jpinto@synopsys.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Pratyush Anand <pratyush.anand@gmail.com>