T4 packet timestamps.

Reference code that shows how to get a packet's timestamp out of
cxgbe(4). Disabled by default because we don't have a standard way
today to pass this information up the stack.

The timestamp is 60 bits wide and each increment represents 1 tick of
the T4's core clock. As an example, the timestamp granularity is ~4.4ns
for this card:

sysctl dev.t4nex.0.core_clock

dev.t4nex.0.core_clock: 228125

MFC after: 1 week