[IA64] ia64: add ar.k0 usage note

Update comment about how ar.k0 is used.  Make the initialization the
same as in start_secondary() (no functional change, just make it look
more similar).

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>