PCI: dwc: Add CONFIG_PCIE_DW_HOST to enable PCI dwc host

Now that PCI designware host has a separate file, add a new PCIE_DW_HOST
config symbol to select the host-only driver. This will enable to
independently select host support and endpoint support (when it's added).

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