spi: Migrate CONFIG_MXC_SPI to Kconfig

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
diff --git a/configs/ge_b450v3_defconfig b/configs/ge_b450v3_defconfig
index 87312ae..855e423 100644
--- a/configs/ge_b450v3_defconfig
+++ b/configs/ge_b450v3_defconfig
@@ -30,5 +30,6 @@
 CONFIG_NETDEVICES=y
 CONFIG_E1000=y
 CONFIG_CMD_E1000=y
+CONFIG_MXC_SPI=y
 CONFIG_OF_LIBFDT=y
 # CONFIG_EFI_LOADER is not set