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 <[email protected]>
Reviewed-by: Alistair Francis <[email protected]>
Tested-by: Alistair Francis <[email protected]>
Message-id: 20180223232233[email protected]
Signed-off-by: Peter Maydell <[email protected]>