Implement DDMI for SFPs

[glennchid]

Need to decide which registers to read and how to display them - is a hidden/expandable sub-block possible?
Add alarms states and tie to block HEALTH fields?

Note, some SFP I2C networks may be behind multiplexers in the PL which will need switching before reading. Work out how to deal with these. see #211

Details of the DDMI interface can be found here: https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://members.snia.org/document/dl/25916&ved=2ahUKEwiR3fSlqcyIAxWRW0EAHZ6oFH8QFnoECAwQAQ&usg=AOvVaw1_H0SPUbcdr31PrbjqvBsM

Useful registers could be:
20 - Vendor
84 - Date of manufacture
92 - DDMI status
110 - SFP status (TX_disabled, RX_LOS etc)
102 - TX power
104 - RX power