net/ice: fix performance for Rx timestamp

net/ice: fix performance for Rx timestamp

In Rx data path, it reads hardware registers per packet, resulting in
big performance drop. This patch improves performance from two aspects:
(1) replace per packet hardware register read by per burst.
(2) reduce hardware register read time from 3 to 2 when the low value of
    time is not close to overflow.

Meanwhile, this patch refines "ice_timesync_read_rx_timestamp" and
"ice_timesync_read_tx_timestamp" API in which
"ice_tstamp_convert_32b_64b" is also used.

Fixes: 953e74e6b73a ("net/ice: enable Rx timestamp on flex descriptor")
Fixes: 646dcbe6c701 ("net/ice: support IEEE 1588 PTP")
Suggested-by: Harry van Haaren <harry.van.haaren@intel.com>
Signed-off-by: Simei Su <simei.su@intel.com>
Acked-by: Qi Zhang <qi.z.zhang@intel.com>