drivers: virt_ivshmem: Allow multiple instances of ivShMem devices.

- Supporting multiple instances of ivShMem virtual devices.
- Introduces DT based configuration for ivShMem devices.
- Add DTS overlay file to test new multiple ivshmem instance capability.
- Enable BDF unspecified device initialization.
  (limited to one instance. An improved version of pcie_bdf_lookup()
  will come soon that fixes this limitation)
- Make PCIE DTS file macros available for a proper ivshmem device
  properties parsing.

Sample for dts file:
pcie0 {
	label = "PCIE_0";
	#address-cells = <1>;
	#size-cells = <1>;
	compatible = "intel,pcie";
	ranges;
    ivshmem0: ivshmem@800 {
	    compatible = "qemu,ivshmem";
	    reg = <PCIE_BDF_NONE PCIE_ID(0x1af4,0x1110)>;
	    label = "IVSHMEM";
	    status = "okay";
    };
};

Signed-off-by: Michael Schmidt <michael1.schmidt@intel.com>