ARM: dts: stih407: Enable PWM nodes only board level

The PWM may not be used on some boards, so enable them only the board file.

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