boards: nrf: uart: Moved UART pin configuration to DTS (nRF boards)

UART pins (TX, RX, RTS, CTS) are now configured in DTS files.
RTS and CTS definitions are optional. If flow control is enabled
and RTS/CTS pins are not defined, then compiler will issue
an error message.

Signed-off-by: Paweł Zadrożniak <pawel.zadrozniak@nordicsemi.no>