bnxt_en: Enable hardware PTP support

Call bnxt_ptp_init() to initialize and register with the clock driver
to enable PTP support.  Call bnxt_ptp_free() to unregister and clean
up during shutdown.

Reviewed-by: Edwin Peer <edwin.peer@broadcom.com>
Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>