spi: Migrate CONFIG_MXS_SPI to Kconfig

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
diff --git a/drivers/spi/Kconfig b/drivers/spi/Kconfig
index a1f9373..686105c 100644
--- a/drivers/spi/Kconfig
+++ b/drivers/spi/Kconfig
@@ -263,6 +263,12 @@
 	help
 	  Enable support for SPI on MPC8XX
 
+config MXS_SPI
+	bool "MXS SPI Driver"
+	help
+	  Enable the MXS SPI controller driver. This driver can be used
+	  on the i.MX23 and i.MX28 SoCs.
+
 config OMAP3_SPI
 	bool "McSPI driver for OMAP"
 	help