input: button_kbd: make driver complementary to gpio buttons

Remove need of dts binding for button keyboard since it reuses
gpio-keys binding. Select gpio-keys driver if button keyboard
is selected since button keyboard can not operate on its own.

Tested-by: Svyatoslav Ryhel <clamor95@gmail.com> # HTC One X T30
Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>