ARM: dts: stm32: fix uart nodes ordering in stm32mp15-pinctrl

Fix usart and uart nodes ordering. Several usart nodes didn't respect
expecting ordering.

Fixes: 077e0638 ("ARM: dts: stm32: Add alternate pinmux for USART2 pins on stm32mp15")

Signed-off-by: Erwan Le Ray <erwan.leray@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>