spi: Migrate CONFIG_MXC_SPI to Kconfig

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
diff --git a/configs/nitrogen6q2g_defconfig b/configs/nitrogen6q2g_defconfig
index a1b3e8e..fa68b53 100644
--- a/configs/nitrogen6q2g_defconfig
+++ b/configs/nitrogen6q2g_defconfig
@@ -40,6 +40,7 @@
 CONFIG_PHY_MICREL=y
 CONFIG_PHY_MICREL_KSZ90X1=y
 CONFIG_NETDEVICES=y
+CONFIG_MXC_SPI=y
 CONFIG_USB=y
 CONFIG_USB_STORAGE=y
 CONFIG_USB_KEYBOARD=y