Input: gpio-keys - request and configure GPIOs

Currently, gpio_keys.c assumes the GPIOs to be already properly configured;
this patch changes gpio-keys to perform explicit calls to gpio_request() and
gpio_configure_input().

This matches the behaviour of leds-gpio.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>