spi: Migrate CONFIG_MXS_SPI to Kconfig

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
diff --git a/configs/mx28evk_auart_console_defconfig b/configs/mx28evk_auart_console_defconfig
index a3590bb..e9f6ff4 100644
--- a/configs/mx28evk_auart_console_defconfig
+++ b/configs/mx28evk_auart_console_defconfig
@@ -41,6 +41,7 @@
 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