bnxt_en: Add driver support to use Real Time Counter for PTP

Add support for RTC mode if it is supported by firmware.  In RTC
mode, the PHC is set to the 64-bit clock.  Because the legacy interface
is 48-bit, the driver still has to keep track of the upper 16 bits and
handle the rollover.

Cc: Richard Cochran <richardcochran@gmail.com>
Reviewed-by: Somnath Kotur <somnath.kotur@broadcom.com>
Signed-off-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>