ARM: zynq: Specify MMC controller number for boot sequence

Xilinx Zynq SoC has two sdhci controllers but boot is only possible from
the first one. That's why there is a need to specify controller number.
mmc1 is supposed to be secondary boot device and should be also listed in
distribution boot.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/board/xilinx/zynq/board.c b/board/xilinx/zynq/board.c
index 35191b2..7cfe69d 100644
--- a/board/xilinx/zynq/board.c
+++ b/board/xilinx/zynq/board.c
@@ -52,7 +52,7 @@
 		env_set("modeboot", "norboot");
 		break;
 	case ZYNQ_BM_SD:
-		mode = "mmc";
+		mode = "mmc0";
 		env_set("modeboot", "sdboot");
 		break;
 	case ZYNQ_BM_JTAG: