ARM: configs: add a gemini defconfig

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 <[email protected]>
Cc: Paulius Zaleckas <[email protected]>
Cc: Hans Ulli Kroll <[email protected]>
Cc: Florian Fainelli <[email protected]>
Signed-off-by: Linus Walleij <[email protected]>
Signed-off-by: Olof Johansson <[email protected]>