powerpc/powernv: Avoid waiting for secondary hold spinloop with OPAL

OPAL boot does not insert secondaries at 0x60 to wait at the secondary
hold spinloop. Instead they are started later, and inserted at
generic_secondary_smp_init(), which is after the secondary hold
spinloop.

Avoid waiting on this spinloop when booting with OPAL firmware. This
wait always times out that case.

This saves 100ms boot time on powernv, and 10s of seconds of real time
when booting on the simulator in SMP.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>