dm: core: add clocks node scan

Currently, all fixed-clock declared in "clocks" node in device tree
can be binded by clk_fixed_rate.c driver only if each of them have
the "simple-bus" compatible string.
This constraint has been invoked here [1].

This patch offers a solution to avoid adding "simple-bus" compatible
string to nodes that are not busses.

[1] https://patchwork.ozlabs.org/patch/558837/



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