mxs: fix warning in SPL with console support

Signed-off-by: Stefano Babic <sbabic@denx.de>
diff --git a/arch/arm/cpu/arm926ejs/mxs/spl_boot.c b/arch/arm/cpu/arm926ejs/mxs/spl_boot.c
index e643ead..d3e1369 100644
--- a/arch/arm/cpu/arm926ejs/mxs/spl_boot.c
+++ b/arch/arm/cpu/arm926ejs/mxs/spl_boot.c
@@ -19,7 +19,9 @@
 
 DECLARE_GLOBAL_DATA_PTR;
 static gd_t gdata __section(".data");
+#ifdef CONFIG_SPL_SERIAL_SUPPORT
 static bd_t bdata __section(".data");
+#endif
 
 /*
  * This delay function is intended to be used only in early stage of boot, where