This fixes a build failure with gpio_keys and CONFIG_GPIOLIB=n (mtx1):
  CC      drivers/input/keyboard/gpio_keys.o
gpio_keys.c: In function 'gpio_keys_report_event':
gpio_keys.c:325:2: error: implicit declaration of function 'gpio_get_value_cansleep'
gpio_keys.c: In function 'gpio_keys_setup_key':
gpio_keys.c:390:3: error: implicit declaration of function 'gpio_set_debounce'

Also add stubs for the other new functions.
Signed-off-by: Manuel Lauss <manuel.lauss@googlemail.com>
To: Linux-MIPS <linux-mips@linux-mips.org>
Cc: Florian Fainelli <florian@openwrt.org>
Cc: Wolfgang Grandegger <wg@grandegger.com>
Patchwork: https://patchwork.linux-mips.org/patch/2346/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>