[POWERPC] Make partitions optional in physmap_of

The latest physmap_of driver has a small error where it will fail the probe
with:

physmap-flash: probe of fff00000.small-flas failed with error -2

if there are no partition subnodes in the device tree and the old style binding
is not used.  Since partition definitions are optional, the probe should still
succeed.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>