[POWERPC] Add uevent handler for of_platform_bus

Adding this handler allow userspace to properly handle the module
autoloading. The generation of the uevent itself is now common to
all bus using of_device, so not much code here.

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>