[POWERPC] Make lparcfg.c work when both iseries and pseries are selected

This also consolidates the initial bits of lparcfg_data() and adds the
partition number to the iSeries flattened device tree.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>