dt-bindings: reset: uniphier: add USB3 core reset support

Add DT bindings for reset control of USB3 core implemented in UniPhier SoCs.
The reset control belongs to USB3 glue layer.

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>