dev, arm: Add support for linux generic pci host driver

dev, arm: Add support for linux generic pci host driver

This change adds support for a generic pci host bus driver that
has been included in recent Linux kernel instead of the more
bespoke one we've been using to date. It also works with
aarch64 so it provides PCI support for 64-bit ARM Linux.

To make this work a new configuration option pci_io_base is added
to the RealView platform that should be set to the start of
the memory used as memory mapped IO ports (IO ports that are
memory mapped, not regular memory mapped IO). And a parameter
pci_cfg_gen_offsets which specifies if the config space
offsets should be used that the generic driver expects.

To use the pci-host-generic device you need to:
pci_io_base = 0x2f000000 (Valid for VExpress EMM)
pci_cfg_gen_offsets = True

and add the following to your device tree:

    pci {
        compatible = "pci-host-ecam-generic";
        device_type = "pci";
        #address-cells = <0x3>;
        #size-cells = <0x2>;
        #interrupt-cells = <0x1>;
        //bus-range = <0x0 0x1>;

        // CPU_PHYSICAL(2)  SIZE(2)
        // Note, some DTS blobs only support 1 size
        reg = <0x0 0x30000000 0x0 0x10000000>;

        // IO (1), no bus address (2), cpu address (2), size (2)
        // MMIO (1), at address (2), cpu address (2), size (2)
        ranges = <0x01000000 0x0 0x00000000 0x0 0x2f000000 0x0 0x10000>,
                 <0x02000000 0x0 0x40000000 0x0 0x40000000 0x0 0x10000000>;

        // With gem5 we typically use INTA/B/C/D one per device
        interrupt-map = <0x0000 0x0 0x0 0x1 0x1 0x0 0x11 0x1
                         0x0000 0x0 0x0 0x2 0x1 0x0 0x12 0x1
                         0x0000 0x0 0x0 0x3 0x1 0x0 0x13 0x1
                         0x0000 0x0 0x0 0x4 0x1 0x0 0x14 0x1>;

        // Only match INTA/B/C/D and not BDF
        interrupt-map-mask = <0x0000 0x0 0x0 0x7>;
    };