ARM: dts: stm32: add usart3 node to stm32mp15xx-dkx boards

Adds usart3 node to stm32mp15xx-dkx and usart3 alias to stm32mp157a-dk1
and stm32mp157c-dk2 boards. usart3 pins are connected to GPIO Expansion
connector. usart3 is disabled by default.

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