spi: Migrate CONFIG_MXC_SPI to Kconfig

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
diff --git a/configs/ot1200_spl_defconfig b/configs/ot1200_spl_defconfig
index d83946f..98314d7 100644
--- a/configs/ot1200_spl_defconfig
+++ b/configs/ot1200_spl_defconfig
@@ -41,6 +41,7 @@
 CONFIG_SPI_FLASH_SST=y
 CONFIG_SPI_FLASH_WINBOND=y
 CONFIG_PHYLIB=y
+CONFIG_MXC_SPI=y
 CONFIG_DM_THERMAL=y
 CONFIG_USB=y
 CONFIG_USB_STORAGE=y