ARM: samsung: allow serial driver to be disabled

If CONFIG_SERIAL_SAMSUNG is disabled, we run into build errors
with some samsung platforms. This adds a couple of #ifdef
statements to hopefully deal with this more gracefully.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Kukjin Kim <kgene.kim@samsung.com>
Cc: Tomasz Figa <tomasz.figa@gmail.com>
Cc: Ben Dooks <ben-linux@fluff.org>