more analog gain options

[se6astian-pi]

Currently the axiom_set_gain.sh script is limited to factors 1-4 but there is a register: ADC_range_mult2 (reg 100) that offer x4, x8 and x16 factors. Currently this register is hardcoded to x8.

Maybe this is on purpose but it would be interesting to verify if the x16 or x4 are not also useful.

Also there is Figure 96 : ADC Range Settings indicating the ADC_Range Setting needs to be adjusted for LVDS speed, maybe this is already incorporated by the custom values used in axiom_set_gain.sh