ARM: dts: stm32: Add missing USART3 pin config to stm32f469-disco board

This patch adds USART3 pin configuration on PB10/PA11 pins
for STM32F469I-DISCO board.

Signed-off-by: Bruno Herrera <bruherrera@gmail.com>
Signed-off-by: Alexandre TORGUE <alexandre.torgue@st.com>