Implement input device interface, rewrite the keypad input to use gpio
callbacks and implement basic polling in main. The input devices add
themselves to the devices list based on custom kernel configs.

Signed-off-by: Luca Seritan <luca.seritan@huawei.com>
Signed-off-by: Grzegorz Gwozdz <grzegorz.gwozdz@huawei.com>