RFC: Capability addition for IEEE 1588

I am adding support into the em driver for PTP, what I would prefer doing is
to add interface capability support: IFCAP_TSYNC or something like that.
The driver will then enable/disable the feature.

Are there other vendor's hardware providing this support such that a
net/if.h capability would be justified?

There was also some other defines that are needed, an ethertype
for instance, I forget what it was offhand but I did notice it wasn't
there yet.

Cheers,

Jack