dt-bindings: Document JZ4770 PHY bindings

Add documentation for the devicetree bindings of the
Ingenic JZ4770 USB transceiver.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200229161820.17824-1-paul@crapouillou.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>