arm64: zynqmp: Add bootcmd_usb4 variable

Add "bootcmd_usb4" variable to boot through usb4 device.

Signed-off-by: Shubhangi Shrikrushna Mahalle <shubhangi.shrikrushna-mahalle@amd.com>
Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/cb36fe6bd1fef540441e1d8c32636ae6f80357b4.1710933852.git.michal.simek@amd.com
diff --git a/board/xilinx/zynqmp/zynqmp_kria.env b/board/xilinx/zynqmp/zynqmp_kria.env
index 7d3ad31..846eceb 100644
--- a/board/xilinx/zynqmp/zynqmp_kria.env
+++ b/board/xilinx/zynqmp/zynqmp_kria.env
@@ -17,6 +17,7 @@
 bootcmd_usb1=devnum=1; run usb_boot
 bootcmd_usb2=devnum=2; run usb_boot
 bootcmd_usb3=devnum=3; run usb_boot
+bootcmd_usb4=devnum=4; run usb_boot
 bootdelay=2
 bootfstype=fat
 bootm_low=0