PCI: dwc: Split pcie-designware.c into host and core files

Split pcie-designware.c into pcie-designware-host.c that contains the host
specific parts of the driver and pcie-designware.c that contains the parts
used by both host driver and endpoint driver.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>