powerpc/pseries: Allow phandle to be specified in formats other than decimal

Allow the phandle passed to the /proc/ppc64/ofdt file to be specified
in formats other than decimal.  This allows us to easily specify phandle
values in hex that would otherwise appear as negative integers.

This is an issue on systems where the value of
/proc/device-tree/ibm,dynamic-reconfiguration-memory.ibm,phandle is
fffffff9.  Having to pass this to the ofdt file as a string results in
a large negative number, and simple_strtoul() does not handle negative
numbers.

Signed-off-by: Nathan Fontenot <nfont@austin.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>