Documentation/driver-model/platform.txt update/rewrite

This is almost a rewrite of the driver-model/platform.txt documentation;
the previous text was obsolete (for several years), evidently it never
got updated to match the change from being a PC "legacy_bus" to the more
widely used core bus for most embedded systems.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>