dm: test: replace dm_scan_dt() by of dm_extended_scan_fdt() in dm_do_test

This allows to scan the DT including all "clocks" node's sub-nodes
in which fixed-clock are defined.
All fixed-clock should be defined inside a clocks node which collect all
external oscillators. Until now, all clocks sub-nodes can't be binded except
if the "simple-bus" compatible string is added which is a hack.

Update test.dts by moving clk_fixed node inside clocks.

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