ARM: gemini: DT for the Cortina Gemini SoC platforms

This adds initial and compulsory device tree support to the
Gemini ARMv4 platform.

We are selecting a bunch of "absolute minimals" for getting a working
system up with just device tree:

- We select USE_OF for natural reasons or nothing works.

- We select CLKSRC_OF and GEMINI_TIMER so we get timekeeping from
  the clocksource.

- We select GPIO_GEMINI because these are used as irqchips, and
  for a generic driver it is not reasonable for those to have to
  select every possible irqchip in the world to work, the platform
  should simply provide the available irqchips.

- We select a UART that can be exprected to work with
  SERIAL_OF_PLATFORM which is the name for an 8250 OF-probed
  serial port.

- We select the syscon-based reset controller: it's not fun when
  "reboot" doesn't work because of Kconfig, so we just select
  POWER_RESET and POWER_RESET_SYSCON.

- We perhaps a bit controversiallt select ARM_APPENDED_DTB, because
  this platform is using the ancient RedBoot, and can *NOT* be
  expected to upgrade its bootloaders. Appended device tree is
  simply how these devices have to work with device tree.

Cc: Janos Laube <janos.dev@gmail.com>
Cc: Paulius Zaleckas <paulius.zaleckas@gmail.com>
Cc: Hans Ulli Kroll <ulli.kroll@googlemail.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>