PCI: rcar: Build pci-rcar-gen2.c only on ARM

The pci-rcar-gen2.c driver (controlled by PCI_RCAR_GEN2) uses struct
pci_sys_data, which only exists on ARM.  Building it on other arches, e.g.,
arm64/shmobile, causes errors like this:

  drivers/pci/host/pci-rcar-gen2.c: In function 'rcar_pci_cfg_base': drivers/pci/host/pci-rcar-gen2.c:112:34: error: dereferencing pointer to incomplete type
    struct rcar_pci_priv *priv = sys->private_data;
                                    ^

Build pci-rcar-gen2.c only on ARM.

[bhelgaas: changelog, split to separate pci-rcar-gen2 from pcie-rcar]
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>