[POWERPC] Souped-up of_platform_device support

This patch first splits of_device.c and of_platform.c, the later containing
the bits relative to of_platform_device's. On the "breaks" side of things,
drivers uisng of_platform_device(s) need to include asm/of_platform.h now
and of_(un)register_driver is now of_(un)register_platform_driver.

In addition to a few utility functions to locate of_platform_device(s),
the main new addition is of_platform_bus_probe() which allows the platform
code to trigger an automatic creation of of_platform_devices for a whole
tree of devices.

The function acts based on the type of the various "parent" devices encountered
from a provided root, using either a default known list of bus types that can be
"probed" or a passed-in list. It will only register devices on busses matching
that list, which mean that typically, it will not register PCI devices, as
expected (since they will be picked up by the PCI layer).

This will be used by Cell platforms using 4xx-type IOs in the Axon bridge
and can be used by any embedded-type device as well.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>