ARM: dts: stm32: Swap USART3 and UART8 alias on STM32MP15xx DHCOM SoM

Swap USART3 and UART8 aliases on STM32MP15xx DHCOM SoM,
make sure UART8 is listed first, USART3 second, because
the UART8 is labeled as UART2 on the SoM pinout, while
USART3 is labeled as UART3 on the SoM pinout.

Fixes: 34e0c784 ("ARM: dts: stm32: Add DH Electronics DHCOM STM32MP1 SoM and PDK2 board")
Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Christoph Niedermaier <cniedermaier@dh-electronics.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>