reset: Add a Gemini reset controller

The Cortina Systems Gemini reset controller is a simple
32bit register with self-deasserting reset lines. It is
accessed using regmap over syscon.

Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>