gpio: ucb1400: Can be built as a module

With the recent code cleanup from Marek Vasut, driver gpio-ucb1400 can
be built as a module, so change symbol GPIO_UCB1400 from bool to
tristate.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Marek Vasut <marex@denx.de>
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>