Input: gpio-keys - allow disabling individual buttons in DT

Add support to disable buttons from DT via status property if given button
is not supported on given platforms. This will help re-using existing dtsi
files across multiple platforms.

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