sandbox: dts: Add compatible string for bind-test node

Usage of lists_bind_fdt() in bind command imposes to add
a compatible string for bind-test node.

Others impacts are:
  - bind-test node is binded at sandbox start, so no need to bind it
    in test_bind_unbind_with_node() test.
  - As explained just above, after sandbox start, now a phy exist.
    In test/dm/phy.c, it was verified that a third phy didn't exist,
    now we must verified that a fourth phy doesn't exist.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>