drivers: mspi_dw: Add support for RX dummy cycles in single line mode

Support for 8 dummy cycles in a single line RX transaction is required
for the standard JEDEC Read SFDP command. The SSI controller does not
support dummy cycles in Standard SPI mode, but the driver can simulate
those by just sending a dummy data byte.

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