ARM: STi: DT: STih407: Abstract common dt nodes into shared files.

The stih410 soc which will be added in the following commit is very similar to
the stih407, to enable maximum re-use of the dt files this commit abstracts the
common parts into a shared dt file stihxxx-b2120 for the board, and also a shared
file stih407-family.dtsi for the SoC.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>