spi: Migrate CONFIG_MXC_SPI to Kconfig

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
diff --git a/configs/mx51evk_defconfig b/configs/mx51evk_defconfig
index a35da58..4eccf51 100644
--- a/configs/mx51evk_defconfig
+++ b/configs/mx51evk_defconfig
@@ -20,6 +20,7 @@
 CONFIG_CMD_DATE=y
 CONFIG_CMD_FAT=y
 CONFIG_ENV_IS_IN_MMC=y
+CONFIG_MXC_SPI=y
 CONFIG_USB=y
 CONFIG_USB_STORAGE=y
 CONFIG_USB_HOST_ETHER=y