ARM: DT: STiH407: Add serial3 pinctrl configuration

Add missing serial 3 pinctrl config. This can be used
on b2206 HVK, where it defaults to PIO31[3] & PIO31[4],
alternate 1.

Signed-off-by: Erwan Le Ray <erwan.leray@st.com>
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: Carmelo Amoroso <carmelo.amoroso@st.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>