powerpc/pmac/smp: Fix CPU hotplug crashes on some machines

On some machines that use i2c to synchronize the timebases (such
as PowerMac7,2/7,3 G5 machines), hotplug CPU would crash when
putting back a new CPU online due to the underlying i2c bus being
closed.

This uses the newly added bringup_done() callback to move the close
along with other housekeeping calls, and adds a CPU notifier to
re-open the i2c bus around subsequent hotplug operations

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>