nxp: sai: add support for passing TX/RX data line through DTS

Some SAI instances are mutliline, meaning they can have multiple
TX/RX data lines (channels). Depending on the board, the index
of the TX/RX data lines that are connected to the consumer
(e.g: the codec) may not always be 0. This commit fixes this
issue by adding support for passing the index of the used
TX/RX data lines through the DTS.

Signed-off-by: Laurentiu Mihalcea <laurentiu.mihalcea@nxp.com>