dm: input: add button_kbd driver

Bootmenu requires an input device with arrows and enter key.
A common smartphone luckily has power, volume up/down buttons,
which may be used for controlling bootmenu.
To use driver, add 'button-kbd' to stdin.

Signed-off-by: Dzmitry Sankouski <dsankouski@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>