update Documentation/driver-model/platform.txt

Make note of the legacy "probe-the-hardware" drivers, and some APIs that
are mostly unused except by such drivers.  We probably can't escape having
legacy drivers for a while (e.g.  old ISA drivers), but we can at least
discourage this style code for new drivers, and unless it's unavoidable.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: Andres Salomon <dilinger@debian.org>
Cc: Dmitry Torokhov <dtor@mail.ru>
Cc: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>