powerpc+of: Move of_drconf_cell struct definition to asm/prom.h

This patch moves the definition of the of_drconf_cell struct to asm/prom.h
to make it available for all powerpc/pseries code.

Signed-off-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>