Implement I2C configuration of clock synthesisers

[glennchid]

Configure Si570 (eg on HGT-x4SFP FMC or Whizz FMC loopback) or Si5338 (on ST1 baseboard) for various clock frequencies; eg 125 Mhz, 124.92 MHz etc.

Si5338 more complicated and may require a script to set all required registers and in the correct sequence).
Si570 simpler (fewer registers and depends on how much frequency is being changed by), and can use the common clock driver in linux, but needs to be visible ot the PS at boot time for this (Si570 may be connected to the PL, eg with the HTG FMC). See #211