mfd: Add twl4030-pwrbutton as a twl4030 child

Make that twl4030-pwrbutton.c driver probe with current
child creation api for twl4030.

Cc: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>