Input: gpio_keys - add hooks to enable/disable device

Allow platform code to specify callbcks that will be invoked when
input device is opened or closed, allowing, for example, to enable
the device.

Signed-off-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>