spi: Migrate CONFIG_MXS_SPI to Kconfig

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
diff --git a/configs/bg0900_defconfig b/configs/bg0900_defconfig
index e097f1c..20c8feb 100644
--- a/configs/bg0900_defconfig
+++ b/configs/bg0900_defconfig
@@ -32,4 +32,5 @@
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_BAR=y
 CONFIG_SPI_FLASH_STMICRO=y
+CONFIG_MXS_SPI=y
 CONFIG_OF_LIBFDT=y