[IA64] pvops: documentation on ia64/pv_ops

Documentation on ia64/pv_ops which describes its strategy and implementation.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Cc: Gerald Pfeifer <gp@novell.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>