spi: Migrate CONFIG_MXC_SPI to Kconfig

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
diff --git a/configs/woodburn_sd_defconfig b/configs/woodburn_sd_defconfig
index ab6f48e..ed18f02 100644
--- a/configs/woodburn_sd_defconfig
+++ b/configs/woodburn_sd_defconfig
@@ -42,3 +42,4 @@
 CONFIG_NAND_MXC=y
 CONFIG_PHYLIB=y
 CONFIG_PHY_MICREL=y
+CONFIG_MXC_SPI=y