of/promtree: no longer call prom_ functions directly; use an ops structure

Rather than assuming an architecture defines prom_getchild and friends,
define an ops struct with hooks for the various prom functions that
pdt.c needs.  This ops struct is filled in by the
arch-(and sometimes firmware-)specific code, and passed to
of_pdt_build_devicetree.

Update sparc code to define the ops struct as well.

Signed-off-by: Andres Salomon <dilinger@queued.net>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>