dt-bindings: Add support for Broadcom USB pin map driver

Add DT bindings for the Broadcom USB pin map driver. This driver allows
some USB input and output signals to be mapped to any GPIO instead
of the normal dedicated pins to/from the XHCI controller.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Al Cooper <alcooperx@gmail.com>
Link: https://lore.kernel.org/r/20201012200007.8862-2-alcooperx@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>