PCI: cadence: Drop unused member from struct cdns_plat_pcie

The struct cdns_plat_pcie contains a member called is_rc that is not
being used beyond being assigned a value within the cdns_plat_pcie_probe()
function, which is then not used for anything.

Thus, drop is_rc from the struct cdns_plat_pcie, especially since there
already is an is_rc member within the struct cdns_plat_pcie_of_data that
is actively used to convey information about the PCIe controller mode.

[kwilczynski: commit log]
Signed-off-by: Li Chen <lchen@ambarella.com>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>