ARM: gemini: don't select SERIAL_OF_PLATFORM

We cannot select the option when SERIAL_8250 is not also set:

warning: (ARCH_GEMINI) selects SERIAL_OF_PLATFORM which has unmet direct dependencies (TTY && HAS_IOMEM && SERIAL_8250 && OF)

This removes the 'select' statement, requiring that users enable the
option manually. Alternatively, we could make it a conditional
'select SERIAL_OF_PLATFORM if SERIAL_8250' or also select a handful
of other symbols that it depends on.

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