driver core: warn that platform_driver_probe can not use deferred probing

Add documentation that platform_driver_probe() is incompatible with
deferred probing.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>