ARM: DT: STiH407: Add serial3 pinctrl configuration

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 <[email protected]>
Signed-off-by: Fabrice Gasnier <[email protected]>
Acked-by: Carmelo Amoroso <[email protected]>
Acked-by: Lee Jones <[email protected]>
Acked-by: Patrice Chotard <[email protected]>
Signed-off-by: Peter Griffin <[email protected]>
Signed-off-by: Maxime Coquelin <[email protected]>