PPC: E500: Set PIR default reset value rather than SPR value

PPC: E500: Set PIR default reset value rather than SPR value

We now reset SPRs to their reset values on CPU reset. So if we want
to have an SPR persistently changed, we need to change its default
reset value rather than the value itself manually.

Do this for SPR_BOOKE_PIR, fixing e500v2 SMP boot.

Reported-by: Frederic Konrad <fred.konrad@greensocs.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
Tested-by: KONRAD Frederic <fred.konrad@greensocs.com>