PCI: dwc: designware: Parse "num-lanes" property in dw_pcie_setup_rc()

The "num-lanes" DT property is parsed in dw_pcie_host_init(). However
num-lanes is applicable to both root complex mode and endpoint mode. As a
first step, move the parsing of this property outside dw_pcie_host_init().
This is in preparation for splitting pcie-designware.c to pcie-designware.c
and pcie-designware-host.c

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