Add DT bindings for Broadcom STB USB EHCI and XHCI drivers.

NOTE: The OHCI driver is not included because it uses the generic
      platform driver.
Signed-off-by: Al Cooper <alcooperx@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20200512150019.25903-3-alcooperx@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>