spi: Migrate CONFIG_MXC_SPI to Kconfig

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
diff --git a/configs/ts4800_defconfig b/configs/ts4800_defconfig
index 98ceaf7..7260149 100644
--- a/configs/ts4800_defconfig
+++ b/configs/ts4800_defconfig
@@ -15,4 +15,5 @@
 CONFIG_CMD_FAT=y
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_PHYLIB=y
+CONFIG_MXC_SPI=y
 CONFIG_OF_LIBFDT=y