ARM: dts: stm32: add pins for usart2/1/4/8 in stm32mp13-pinctrl

Add pins for uart4, uart8, usart1 and usart2 in stm32mp13-pinctrl.dtsi
Theses pins have three states: default, sleep and idle.

Signed-off-by: Valentin Caron <valentin.caron@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>