PCI: rcar: Build pcie-rcar.c only on ARM

The pcie-rcar.c driver (controlled by PCI_RCAR_GEN2_PCIE) uses struct
pci_sys_data and pci_ioremap_io(), which only exist on ARM.  Building it on
other arches, e.g., arm64/shmobile, causes errors like this:

  drivers/pci/host/pcie-rcar.c:138:52: warning: 'struct pci_sys_data' declared inside parameter list
  drivers/pci/host/pcie-rcar.c:380:4: error: implicit declaration of function 'pci_ioremap_io' [-Werror=implicit-function-declaration]

Build pcie-rcar.c only on ARM.

[bhelgaas: changelog, split to separate pci-rcar-gen2 from pcie-rcar]
Reported-by: Wolfram Sang <wsa@the-dreams.de> (pci_ioremap_io())
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>