net: dsa: mv88e6xxx: Remove bogus Kconfig dependency.

The mv88e6xxx is a DSA driver, and it implements DSA style time
stamping of PTP frames.  It has no need of the expensive option to
enable PHY time stamping.  Remove the bogus dependency.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Acked-by: Brandon Streiff <brandon.streiff@ni.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>