of/platform: Move platform devices under /sys/devices/platform

Currently the devices created by drivers/of/platform.c get created at
the root of /sys/devices. This goes against the typical pattern for
sysfs where the top level /sys/devices structure contains categories of
devices, and the structure of devices is placed below that. To fix this,
make the code in drivers/of/platform.c follow the drivers/base/platform.c
behaviour, and use &platform_bus as the default parent for all new
platform_devices and amba_devices.

This change has been discussed for a long time, but nobody has actually
acted on it. Userspace code that expects to find devices under a fixed
/sys/devices/... path will be affected. It isn't /supposed/ to do that,
but if anyone complains then I'll add a default-off workaround option to
put them back into the root.

Signed-off-by: Grant Likely <grant.likely@linaro.org>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>