spi: Migrate CONFIG_MXC_SPI to Kconfig

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
diff --git a/configs/dh_imx6_defconfig b/configs/dh_imx6_defconfig
index 2d047e0..9fbc47e 100644
--- a/configs/dh_imx6_defconfig
+++ b/configs/dh_imx6_defconfig
@@ -49,6 +49,7 @@
 CONFIG_PHY_MICREL_KSZ90X1=y
 CONFIG_NETDEVICES=y
 CONFIG_FEC_MXC=y
+CONFIG_MXC_SPI=y
 CONFIG_USB=y
 CONFIG_USB_STORAGE=y
 CONFIG_USB_GADGET=y