Add dp_length field in application header.

libraries/io/eth/eth.peripheral.yaml

libraries/io/eth/src/vhdl/eth_tester.vhd

libraries/io/eth/src/vhdl/eth_tester_pkg.vhd

libraries/io/eth/src/vhdl/eth_tester_rx.vhd

libraries/io/eth/src/vhdl/eth_tester_tx.vhd

libraries/io/eth/tb/vhdl/tb_eth_tester.vhd