of/net: Move of_get_mac_address() to a common source file.

There are two identical implementations of of_get_mac_address(), one
each in arch/powerpc/kernel/prom_parse.c and
arch/microblaze/kernel/prom_parse.c.  Move this function to a new
common file of_net.{c,h} and adjust all the callers to include the new
header.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
[grant.likely@secretlab.ca: protect header with #ifdef]
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>