ARM: configs: add a gemini defconfig

It makes sense to have a stripped-down defconfig for just Gemini, as
it is a pretty small platform used in NAS etc, and will use appended
device tree. It is also quick to compile and test. Hopefully this
defconfig can be a good base for distributions such as OpenWRT.

I plan to add in the config options needed for the different
variants of Gemini as we go along.

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>
Signed-off-by: Olof Johansson <olof@lixom.net>