gpio: GPIO_REGMAP: select REGMAP instead of depending on it

REGMAP is a hidden (not user visible) symbol. Users cannot set it
directly thru "make *config", so drivers should select it instead of
depending on it if they need it.

Consistently using "select" or "depends on" can also help reduce
Kconfig circular dependency issues.

Therefore, change the use of "depends on REGMAP" to "select REGMAP".

Fixes: ebe36319 ("gpio: add a reusable generic gpio_chip using regmap")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Michael Walle <michael@walle.cc>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Bartosz Golaszewski <brgl@bgdev.pl>
Cc: linux-gpio@vger.kernel.org
Acked-by: Michael Walle <michael@walle.cc>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>