ethdev: add API to read device clock

ethdev: add API to read device clock

Add rte_eth_read_clock to read the raw clock of a device.

The main use is to get the device clock conversion co-efficients to be
able to translate the raw clock of the timestamp field of the pkt mbuf
to a local synced time value.

This function was missing to allow users to convert the Rx timestamp
field to real time without the complexity of the rte_timesync* facility.
One can derivate the clock frequency by calling twice read_clock and
then keep a common time base.

Signed-off-by: Tom Barbette <barbette@kth.se>
Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>