spi: Migrate CONFIG_MXC_SPI to Kconfig

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
diff --git a/configs/cm_fx6_defconfig b/configs/cm_fx6_defconfig
index 5b5280f..183ad42 100644
--- a/configs/cm_fx6_defconfig
+++ b/configs/cm_fx6_defconfig
@@ -62,6 +62,7 @@
 CONFIG_PHYLIB=y
 CONFIG_DM_PMIC=y
 CONFIG_DM_REGULATOR=y
+CONFIG_MXC_SPI=y
 CONFIG_USB=y
 CONFIG_DM_USB=y
 CONFIG_USB_STORAGE=y