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/ch7301.c b/board/gdsys/common/ch7301.c
index 1234149..5e42467 100644
--- a/board/gdsys/common/ch7301.c
+++ b/board/gdsys/common/ch7301.c
@@ -6,6 +6,8 @@
 
 /* Chrontel CH7301C DVI Transmitter */
 
+#ifdef CONFIG_GDSYS_LEGACY_DRIVERS
+
 #include <common.h>
 #include <asm/io.h>
 #include <errno.h>
@@ -61,3 +63,5 @@
 
 	return 0;
 }
+
+#endif /* CONFIG_GDSYS_LEGACY_DRIVERS */