drivers: mspi_dw: Add support for DQS line

Add possibility of using data strobe signal (DQS) called RXDS in
the SSI documentation. This signal may be necessary to make use
of DDR modes.
Remove also a TODO comment that should have been removed when support
for DDR was added in fc64fec8.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>