Fiddling with "struct mode_def imx477_modes[]" when you already have the rest of the header file would be the obvious thing to do and would probably work out fine but that is mostly trial-and-error and not a proper method.
Well you need a datasheet from your sensor’s vendor. There are whole description of the register and initialization sequence. Few hundred values. And from these values you can construct your own register map for the sensor in the way as it was done for supported sensors. For popular sensors datasheets can be obtained just by searching.
