[POWERPC] Add a unified uevent handler for bus based on of_device

This common uevent handler allow the several bus types based on
of_device to generate the uevent properly and avoiding
code duplication.

This handlers take a struct device as argument and can therefore
be used as the uevent call directly if no special treatment is
needed for the bus.

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