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
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