gpio-lynxpoint: Allow building as a module

Change CONFIG_GPIO_LYNXPOINT from bool to tristate so that the
gpio-lynxpoint driver can be built as a module.

Add the required glue: an exit function to unregister the driver, and
module information.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Mathias Nyman <mathias.nyman@linux.intel.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>