Several core device drivers maintain a copy of some device state.
Since this copy is not transferred between experiments this backing state
can be different from device state when a new experiment is started.
This commit adds support for injecting initial backing state into
experiments via the device database and sets initial backing state where
known.
ad53xx (zotino): spi2 xfer_duration
novogorny: pgia gains
sampler: pgia gains, spi2 pgia and adc xfer_duration
suservo: pgia gains, spi2 pgia xfer_duration
urukul: cpld cfg (partial: rf_sw), attenuator register
spi2: div/length for xfer_duration
close#1003
