Input: gpio-keys - add shutdown callback

On some platforms (e.g.: ARCH_BRCMSTB) it is possible to enter
"poweroff" while leaving some wake-up sources enabled such as key
presses in order to allow for the system to wake-up.

Wire up a .shutdown() callback which calls into the existing
gpio_keys_suspend() since the logic is essentially the same.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>