userspace/snmpd: add counters of PTP tx and rx packets to wrsPortStatusTable

Add wrsPortStatusPtpTxFrames and wrsPortStatusPtpRxFrames to wrsPortStatusTable

Additionally:
--follow pointer to pp_instances in snmp_shmem.c
--update MIB
--uplift ppsi
--shmem dump with new variables (ptp_tx_count and ptp_rx_count)

Signed-off-by: Adam Wujek <adam.wujek@cern.ch>