irqchip/gemini: Refactor Gemini driver to reflect Faraday origin

The Gemini irqchip turns out to be a standard IP component from
Faraday Technology named FTINTC010 after some research and new
information.

- Rename the driver and all symbols to reflect the new information.
- Add the new compatible string "faraday,ftintc010"
- Create a Kconfig symbol CONFIG_FARADAY_FTINTC010 so that SoCs
  using this interrupt controller can easily select and reuse it
  instead of hardwiring it to ARCH_GEMINI

I have created a separate patch to select the new Kconfig symbol
from the Gemini machine, which will be merged through the ARM
SoC tree.

Cc: Greentime Hu <green.hu@gmail.com>
Cc: Paulius Zaleckas <paulius.zaleckas@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>