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/common/fpga.c b/board/gdsys/common/fpga.c
index f189e5f..5ba6613 100644
--- a/board/gdsys/common/fpga.c
+++ b/board/gdsys/common/fpga.c
@@ -4,6 +4,8 @@
  * Dirk Eibach,  Guntermann & Drunck GmbH, dirk.eibach@gdsys.cc
  */
 
+#ifdef CONFIG_GDSYS_LEGACY_DRIVERS
+
 #include <common.h>
 #include <gdsys_fpga.h>
 
@@ -22,3 +24,5 @@
 
 	return 0;
 }
+
+#endif /* CONFIG_GDSYS_LEGACY_DRIVERS */