Driver core: export platform_device_register_data as a GPL symbol

This allows MFD's to register/bind drivers for their sub devices while
still being compiled as a module.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>