ata: Add DT bindings for the Gemini SATA bridge

This adds device tree bindings for the Cortina Systems Gemini
PATA to SATA bridge.

Cc: devicetree@vger.kernel.org
Cc: John Feng-Hsin Chiang <john453@faraday-tech.com>
Cc: Greentime Hu <green.hu@gmail.com>
Acked-by: Hans Ulli Kroll <ulli.kroll@googlemail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Tejun Heo <tj@kernel.org>