ARM: STi: DT: STih407: Re-order #include <*.dtsi> files

This patch fixes a regression where serial is enabled by the first
(board) DTSI, then disabled by the second (SoC) file.  To enable
serial and keep it enabled, we need to include the file which enables
it last.

Reported-by: LAVA [via Peter Griffin <peter.griffin@linaro.org&gt;]>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>