nvidia: Add XUSB firmware for Tegra186

Add a firmware file for the XUSB controller found on Tegra186.
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Josh Boyer <jwboyer@kernel.org>

WHENCE

@@ -3691,6 +3691,9 @@ Version: v45.46
 File: nvidia/tegra210/xusb.bin
 Version: v50.10
 
+File: nvidia/tegra186/xusb.bin
+Version: v55.15
+
 Licence: Redistributable. See LICENCE.nvidia for details
 
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