dm: test: Add a new test case for dm_test_eth_rotate

Add one more ethernet device node in the sandbox test device tree,
with name 'sbe5'. This is to support a new test case for testing
network device rotation.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>