Revert "powerpc: Secondary CPUs must set cpu_callin_map after setting active and online"

This reverts commit 7c5c92ed.

Although this did fix the bug it was aimed at, it also broke secondary
startup on platforms that use give/take_timebase(). Unfortunately we
didn't detect that while it was in next.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>