spi: ti_qspi: Add support for CS other than CS0

Make sure corresponding setup registers are updated depending on CS.
This ensures that driver can support QSPI flashes on ChipSelects other
than on CS0

Reported-by: Andreas Dannenberg <dannenberg@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>