ppc4xx: Fix acadia_nand build problem

Don't include testdram() on NAND-booting target acadia_nand. This saves
a few bytes and makes the target build clean again.

Signed-off-by: Stefan Roese <sr@denx.de>
diff --git a/board/amcc/acadia/memory.c b/board/amcc/acadia/memory.c
index 9346d2c..709d41e 100644
--- a/board/amcc/acadia/memory.c
+++ b/board/amcc/acadia/memory.c
@@ -117,7 +117,9 @@
 	return (CFG_MBYTES_RAM << 20);
 }
 
+#ifndef CONFIG_NAND_SPL
 int testdram(void)
 {
 	return (0);
 }
+#endif