spi: Migrate CONFIG_MXS_SPI to Kconfig

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
diff --git a/configs/mx28evk_nand_defconfig b/configs/mx28evk_nand_defconfig
index 3b9962f..8d8bafa 100644
--- a/configs/mx28evk_nand_defconfig
+++ b/configs/mx28evk_nand_defconfig
@@ -40,6 +40,7 @@
 CONFIG_NAND_MXS=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_SST=y
+CONFIG_MXS_SPI=y
 CONFIG_USB=y
 CONFIG_USB_EHCI_HCD=y
 CONFIG_USB_STORAGE=y