spi: Migrate CONFIG_MXC_SPI to Kconfig

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
diff --git a/configs/flea3_defconfig b/configs/flea3_defconfig
index 923d2ac..50462b4 100644
--- a/configs/flea3_defconfig
+++ b/configs/flea3_defconfig
@@ -27,6 +27,7 @@
 CONFIG_NAND_MXC=y
 CONFIG_PHYLIB=y
 CONFIG_PHY_MICREL=y
+CONFIG_MXC_SPI=y
 CONFIG_OF_LIBFDT=y
 CONFIG_FDT_FIXUP_PARTITIONS=y
 # CONFIG_EFI_LOADER is not set