dt-bindings: usb: add bindings doc for HiSilicon STB xHCI host controller

This commit adds bindings doc for HiSilicon STB xHCI host controller.

Signed-off-by: Jianguo Sun <sunjianguo1@huawei.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>