platform: add new device registration helper

Add a helper that registers simple platform_device w/o resources but with
parent and device data.

This is usefull to cleanup platform code from code that registers such
simple devices as leds-gpio, generic-bl, etc.

Signed-off-by: Dmitry Baryshkov <dbaryshkov@gmail.com>
Cc: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>