spi: Migrate CONFIG_MXC_SPI to Kconfig

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
diff --git a/configs/marsboard_defconfig b/configs/marsboard_defconfig
index d00116e..76b55df 100644
--- a/configs/marsboard_defconfig
+++ b/configs/marsboard_defconfig
@@ -22,6 +22,7 @@
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_SST=y
 CONFIG_PHYLIB=y
+CONFIG_MXC_SPI=y
 CONFIG_DM_THERMAL=y
 CONFIG_USB=y
 CONFIG_USB_STORAGE=y