spi: Migrate CONFIG_MXC_SPI to Kconfig

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
diff --git a/configs/woodburn_defconfig b/configs/woodburn_defconfig
index fbc5178..175240d 100644
--- a/configs/woodburn_defconfig
+++ b/configs/woodburn_defconfig
@@ -31,3 +31,4 @@
 CONFIG_NAND_MXC=y
 CONFIG_PHYLIB=y
 CONFIG_PHY_MICREL=y
+CONFIG_MXC_SPI=y