NTB: switchtec_ntb: Add link management

switchtec_ntb checks for a link by looking at the shared memory
window. If the magic number is correct and the other side indicates
their link is enabled then we take the link to be up.

Whenever we change our local link status we send a msg to the
other side to check whether it's up and change their status.

The current status is maintained in a flag so ntb_is_link_up
can return quickly.

We utilize Switchtec's link status notifier to also check link changes
when the switch notices a port changes state.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Reviewed-by: Stephen Bates <sbates@raithlin.com>
Reviewed-by: Kurt Schwemmer <kurt.schwemmer@microsemi.com>
Acked-by: Allen Hubbe <Allen.Hubbe@dell.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>