[POWERPC] Fix cell's mmio nvram to properly parse device tree

The mmio nvram driver (used by cell only atm) isn't properly parsing
the device-tree, meaning that nvram isn't found correctly on the new
Cell blades.  It works ok for old blades where the nvram is at the
root of the device tree but fails on Malta and CAB when it's hanging
off axon.  This fixes it by using the proper OF parsing functions.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>