[POWERPC] iSeries: Make iseries_reg_save private to iSeries

Now that we have the alpaca, the reg_save_ptr is no longer needed in the
paca.  Eradicate all global uses of it and make it static in the iSeries
lpardata.c

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