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>

drivers/gpio/Kconfig

@@ -703,7 +703,7 @@ config GPIO_74X164
 comment "AC97 GPIO expanders:"
 
 config GPIO_UCB1400
-	bool "Philips UCB1400 GPIO"
+	tristate "Philips UCB1400 GPIO"
 	depends on UCB1400_CORE
 	help
 	  This enables support for the Philips UCB1400 GPIO pins.