ARM: dts: stm32: add uart nodes and uart aliases on stm32mp135f-dk

ARM: dts: stm32: add uart nodes and uart aliases on stm32mp135f-dk

Update device-tree stm32mp135f-dk.dts to add usart1, uart8, usart2
and uart aliases.

- Usart2 is used to interface a BT device, enable it by default.
- Usart1 and uart8 are available on expansion connector.
  They are kept disabled. So, the pins are kept in analog state to
  lower power consumption by default or can be used as GPIO.
- Uart4 is used for console.

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