ARM: dts: stm32: add uart7 support to stm32mp15xx-dkx boards

Adds uart7 node to stm32mp15xx-dkx and uart7 alias to stm32mp157a-dk1 and
stm32mp157c-dk2 boards. uart7 pins are connected to Arduino connector.
uart7 is disabled by default.

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