boards: Harmonized UART config of Nucleo-64 boards

- nucleo-f334r8:
  - Added USART1 and USART3 config
- nucleo-f401re:
  - Changed order of UARTs to match the other boards
- nucleo-f410rb:
  - Added USART6 configuration
  - Fixed incorrect DMA settings (according to reference manual of the MCU)
- nucleo-f446re:
  - Changed order of UARTs to match the other boards
- nucleo-l152re:
  - Changed order of UARTs to match the other boards

The result of this harmonization is, that the first UART is always USART2, the
other USARTs are added in ascending order.