gdsys: Introduce GDSYS_LEGACY_DRIVERS

Future gdsys boards will switch from the legacy drivers in board/gdsys/common
to DM-based drivers.

Define a Kconfig option that disables the legacy drivers.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
diff --git a/board/gdsys/mpc8308/mpc8308.c b/board/gdsys/mpc8308/mpc8308.c
index a53135b..ae77fc2 100644
--- a/board/gdsys/mpc8308/mpc8308.c
+++ b/board/gdsys/mpc8308/mpc8308.c
@@ -24,6 +24,7 @@
 
 DECLARE_GLOBAL_DATA_PTR;
 
+#ifdef CONFIG_GDSYS_LEGACY_DRIVERS
 /* as gpio output status cannot be read back, we have to buffer it locally */
 u32 gpio0_out;
 
@@ -116,3 +117,4 @@
 
 	return 0;
 }
+#endif