Input: gpio_keys - clean up device tree parser

- fix sizeof in memset;
- clean up dt properties extraction;
- use for_each_chil_of_node macro;
- use of_get_child_count();
- use of_match_ptr macro.

Signed-off-by: Alexandre Pereira da Silva <aletes.xgr@gmail.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>
[Fabio Estevam <fabio.estevam@freescale.com>: fix fix NULL pointer
dereference for dt case - pdata->buttons wasn't initialized]
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>