ARM: dts: stm32: clean uart aliases on stm32mp15xx-exx boards

Remove duplicates and clean uart aliases.
Uart aliases and uart pins should be declared and associated to
uart instance at the same time.

Put also aliases node above chosen node as same as stm32mp157c-dk2.dts.

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