configs: imx: Use CONFIG_SF_DEFAULT_CS

CONFIG_SPI_FLASH_CS is not used anywhere.

Use CONFIG_SF_DEFAULT_CS instead.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>
Acked-by: Stefano Babic <sbabic@denx.de>