spi: mxc_spi: Fix chipselect on DM_SPI driver uclass

CS GPIO activation low/high is determinated by the device tree
so we don't need to take in accoung in cs_activate and cs_deactivate

Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>