arm64: dts: darcy: enable xusb in bootloader

arm64: dts: darcy: enable xusb in bootloader

- added flag 'nvidia,bootloader-xusb-enable' in /chosen for xusb support
  in BL
- bootloader will check it to init xhci controller or not

Bug 200140080

Change-Id: I09bc6d06efad6e018c194a3a35d2eb3b9a8800f5
Signed-off-by: Yunfan Zhang <yunfanz@nvidia.com>
Reviewed-on: http://git-master/r/1020392
Reviewed-by: Automatic_Commit_Validation_User
GVS: Gerrit_Virtual_Submit
Reviewed-by: Martin Gao <marting@nvidia.com>
Reviewed-by: Vinayak Pane <vpane@nvidia.com>