Implement dummy port which can be requested by setting appropriate bit
in probe_ent->dummy_port_mask.  The dummy port is used as placeholder
for stolen legacy port.  This allows libata to guarantee that
index_of(ap) == ap->port_no == actual_device_port_no, and thus to
remove error-prone ap->hard_port_no.

As it's used only when one port of a legacy controller is reserved by
some other entity (e.g. IDE), the focus is on keeping the added *code*
complexity at minimum, so dummy port allocates all libata core
resources and acts as a normal port.  It just has all dummy port_ops.

This patch only implements dummy port.  The following patch will make
libata use it for stolen legacy ports.
Signed-off-by: NTejun Heo <htejun@gmail.com>