PCI: dwc: Remove dependency of designware on CONFIG_PCI

CONFIG_PCI is used to enable host mode PCI. In preparation for adding
endpoint mode support to designware driver, remove the dependency of
designware on CONFIG_PCI and make only the host-specific part depend on
CONFIG_PCI.

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