Input: gpio-keys - allow setting input device name in DT

Allow specifying name if input device via device tree property. This helps
userspace code to get name and perform proper event to key mapping in some
cases (for example, on Android).

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>