spi: Migrate CONFIG_MPC8XXX_SPI to Kconfig

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
diff --git a/drivers/spi/Kconfig b/drivers/spi/Kconfig
index e7e1ccd..a16faa6 100644
--- a/drivers/spi/Kconfig
+++ b/drivers/spi/Kconfig
@@ -263,6 +263,11 @@
 	help
 	  Enable support for SPI on MPC8XX
 
+config MPC8XXX_SPI
+	bool "MPC8XXX SPI Driver"
+	help
+	  Enable support for SPI on the MPC8XXX PowerPC SoCs.
+
 config MXC_SPI
 	bool "MXC SPI Driver"
 	help