target-ppc: ppce500_spin.c uses SPR_PIR, should use SPR_BOOKE_PIR

ppce500_spin.c uses SPR_PIR to initialize the spin table, however on
Book E processors the correct SPR is SPR_BOOKE_PIR.

Signed-off-by: Aaron Larson <alarson@ddci.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
