ARM: gemini: delete all boardfiles

Delete the Gemini boardfiles: we have corresponding, fully-featured
device trees for all these boards. Delete the referenced include
files. Delete the local config symbols, especially one for
"swapped memory", as all supported boards have swapped memory, and
would a new board be supported this is likely not the right way
to achieve it anyways. Only the Kconfig options in the central
arch/arm/Kconfig remains.

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>