powerpc/smp: Lookup avail once per device tree node

The of_device_is_available() check only needs to be done once per device
node, there's no need to repeat it for each thread. Move it out of the
loop.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231229120107.2281153-3-mpe@ellerman.id.au