tsec: Add TSEC_FIBER flag

The TSEC_FIBER flag should be set when a PHY is operating with an
external fiber interface.  Currently it is only used to notify a user
that the PHY is operating in fiber mode.

A short description was also added to the other TSEC flag defines so
that it is clear how they differ from one another.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>