Add device tree support for twl4030 power button driver.

Adding device tree support involved converting the
driver to module_platform_driver().
Signed-off-by: Sebastian Reichel <sre@debian.org>
Acked-by: Kumar Gala <galak@codeaurora.org>
Tested-by: Florian Vaussard <florian.vaussard@epfl.ch>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>