of: make of_find_device_by_node generic

There's no need for this function to be architecture specific and all four
architectures defining it had the same definition.  The function has been
moved to drivers/of/platform.c.

Signed-off-by: Jonas Bonn <jonas@southpole.se>
[grant.likely@secretlab.ca: moved to drivers/of/platform.c, simplified code, and added kerneldoc comment]
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: David S. Miller <davem@davemloft.net>