spi: Add SPI_TX_DUAL mode

Added SPI_TX_DUAL mode flag.

Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Mugunthan V N <mugunthanvnm@ti.com>
Tested-by: Jagan Teki <jteki@openedev.com>
Signed-off-by: Jagan Teki <jteki@openedev.com>