spi: mpc8xx: Add GPIO dependency

Since commit 773ad4ebb1d6 ("spi, mpc8xx: Add support for chipselect via
GPIO and fixups"), DM_GPIO is required for 8xx SPI.

Add the missing dependency to avoid build failures.

Fixes: 773ad4ebb1d6 ("spi, mpc8xx: Add support for chipselect via GPIO and fixups")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
diff --git a/drivers/spi/Kconfig b/drivers/spi/Kconfig
index 69b184b..6124346 100644
--- a/drivers/spi/Kconfig
+++ b/drivers/spi/Kconfig
@@ -262,7 +262,7 @@
 
 config MPC8XX_SPI
 	bool "MPC8XX SPI Driver"
-	depends on MPC8xx
+	depends on MPC8xx && DM_GPIO
 	help
 	  Enable support for SPI on MPC8XX