Is it necessary that the nodes shown in "lspci" output, have to be defined in device tree ? I have device tree file for P2041RDB. In that only one node is created for pci i.e. bus 0. Now I have customized the reference board and connected one pcie-pci bridge to bus 0 annd four pci devices are connected to that bridge. So in my lspci output, six node is showing. So is it necessary that I have to declare it all in device tree.
No, peripherals connected to the PCI-bus doesn't need to be in the DTS file, as they can be enumerated during runtime.
Peripherals sitting on non-enumerable buses, OTOH, needs to be added to the DTS file. This could be peripherals on the memory bus, I2C, SPI, etc.