i2c: move twl4030-pwrbutton to child registration style

This patch is *compile tested only*. It moves twl4030-pwrbutton
to a platform_driver so it can be registered as a child of
twl4030-core.c.

Cc: David Brownell <dbrownell@users.sourceforge.net>
Cc: Samuel Ortiz <sameo@openedhand.com>
Signed-off-by: Felipe Balbi <me@felipebalbi.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>