spi: Migrate CONFIG_MXC_SPI to Kconfig

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
diff --git a/configs/cgtqmx6eval_defconfig b/configs/cgtqmx6eval_defconfig
index 0d2e555..f7e1687 100644
--- a/configs/cgtqmx6eval_defconfig
+++ b/configs/cgtqmx6eval_defconfig
@@ -50,6 +50,7 @@
 CONFIG_DFU_MMC=y
 CONFIG_DFU_SF=y
 CONFIG_PHYLIB=y
+CONFIG_MXC_SPI=y
 CONFIG_USB=y
 CONFIG_USB_STORAGE=y
 CONFIG_USB_KEYBOARD=y