Add a reset line to enable USB3 core implemented in UniPhier SoCs.

This reuses only the reset operations in reset-simple, because
the reset-simple doesn't handle any SoC-dependent clocks and resets.
This reset line is included in the USB3 glue layer, and it's necessary
to enable clocks and deassert resets of the layer before using this
reset line.
Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>