xilinx_spips: Enable only two slaves when reading/writing with stripe

xilinx_spips: Enable only two slaves when reading/writing with stripe

Assert only the lower cs on bus 0 and upper cs on bus 1 when both buses and
chip selects are enabled (e.g reading/writing with stripe).

Signed-off-by: Francisco Iglesias <frasse.iglesias@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Tested-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: 20180223232233.31482-2-frasse.iglesias@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>