ARM64: zynqmp: Allow overwrite identification string

Keep default option there but allow overwrite it.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/include/configs/xilinx_zynqmp.h b/include/configs/xilinx_zynqmp.h
index dcd7552..5008722 100644
--- a/include/configs/xilinx_zynqmp.h
+++ b/include/configs/xilinx_zynqmp.h
@@ -35,7 +35,9 @@
 /* Cache Definitions */
 #define CONFIG_SYS_CACHELINE_SIZE	64
 
-#define CONFIG_IDENT_STRING		" Xilinx ZynqMP"
+#if !defined(CONFIG_IDENT_STRING)
+# define CONFIG_IDENT_STRING		" Xilinx ZynqMP"
+#endif
 
 #define CONFIG_SYS_INIT_SP_ADDR		(CONFIG_SYS_SDRAM_BASE + 0x7fff0)