soc: Add SoC driver for Gemini

This adds an SoC driver for the Gemini. Currently there
is only one thing not fitting into any other framework,
and that is the bus arbitration setting.

All Gemini vendor trees seem to be setting this register to
exactly the same arbitration so we just add a small code
snippet to do this at subsys_init() time before any other
drivers kick in.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>