PciHostBridgeFdtDxe: factor out ECAM-specific portions into lib

[andreiw]

Will be necessary to use this driver on non-ecam platforms. This basically should be an init/fini pair + function to read/write a config space register. Also add a VOID *Context to _PCI_ROOT_BRIDGE_INSTANCE so the lib can maintain extra state if needed.

All state manipulated by init/fini/cfg accessors should be pushed into a common struct, i.e.:

  UINT32                                              Segment;
  EFI_DT_REG                                          ConfigReg;
  UINT64                                              Attributes;
  UINT64                                              Supports;
  EFI_DT_RANGE                                        BusRange;
  EFI_DT_RANGE                                        IoRange;
  EFI_DT_RANGE                                        MemRange;
  EFI_DT_RANGE                                        PMemRange;
  EFI_DT_RANGE                                        MemAbove4GRange;
  EFI_DT_RANGE                                        PMemAbove4GRange;
  BOOLEAN                                             DmaAbove4G;
  BOOLEAN                                             NoExtendedConfigSpace;

Originally posted by @andreiw in #12 (comment)