spi: Migrate CONFIG_MXC_SPI to Kconfig

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
diff --git a/configs/mx31pdk_defconfig b/configs/mx31pdk_defconfig
index a186a12..26650ef 100644
--- a/configs/mx31pdk_defconfig
+++ b/configs/mx31pdk_defconfig
@@ -23,3 +23,4 @@
 CONFIG_SMC911X=y
 CONFIG_SMC911X_BASE=0xB6000000
 CONFIG_SMC911X_32_BIT=y
+CONFIG_MXC_SPI=y