Bug #1230: Added tc/read_serdes_phy.py and modules/mmd_serdes.py to read SERDES PHY registers for diagnosing errors in case serdes.py fails.

1) If --diag_sync != 0 to run --diag_mode= 'tx_rx', 'rx', or 'local' for another interval time than 1 sec. After the test interval the external PPS is enabled again with write_cr_syncon.
2) For mode --diag_mode = 'tx'. The external PPS can be enabled again by running the test case again but then with --diag_mode = 'off' (i.e. to stop the tx only test).

Bug #1230: Default --diag_sync=0 to use external sync (= PPS) for serdes.py instead of on board alternative sync.