spi: Migrate CONFIG_MXC_SPI to Kconfig

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
diff --git a/configs/aristainetos2_defconfig b/configs/aristainetos2_defconfig
index 3002fd4..3f91d40 100644
--- a/configs/aristainetos2_defconfig
+++ b/configs/aristainetos2_defconfig
@@ -41,6 +41,7 @@
 CONFIG_PHY_MICREL=y
 CONFIG_PHY_MICREL_KSZ90X1=y
 CONFIG_NETDEVICES=y
+CONFIG_MXC_SPI=y
 CONFIG_USB=y
 CONFIG_USB_STORAGE=y
 CONFIG_VIDEO=y