spi: Migrate CONFIG_MXC_SPI to Kconfig

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
diff --git a/configs/mx6slevk_spinor_defconfig b/configs/mx6slevk_spinor_defconfig
index 6370c8c..ddbdc4c 100644
--- a/configs/mx6slevk_spinor_defconfig
+++ b/configs/mx6slevk_spinor_defconfig
@@ -42,6 +42,7 @@
 CONFIG_DM_REGULATOR_PFUZE100=y
 CONFIG_DM_REGULATOR_FIXED=y
 CONFIG_DM_REGULATOR_GPIO=y
+CONFIG_MXC_SPI=y
 CONFIG_DM_THERMAL=y
 CONFIG_USB=y
 CONFIG_DM_USB=y