bnxt_en: Event handler for PPS events

Once the PPS pins are configured, the FW can report
PPS values using ASYNC event. This patch adds the
ASYNC event handler and subsequent reporting of the
events to kernel.

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>