Hi,
A start might be to have a look in your kernel config to see if there's anything available in your specific build. In my iMX8 menuconfig I have two FM drivers, Silicon laboratories 4761/64/68 AM/FM radio and the TI WL1273 FM radio.
These are found in Device Drivers > Multifunction device drivers. Often peripherals have similar industry standard interfaces and drivers can be adapted for different families of devices. Might be worth seeing what you have already in your build, get familiar with the drivers and see what chip sets are available that may come close 'code wise'. If you have a FM chipset in mind see is there is driver source for it and try building it (with a few mods probably) and probe it in and you never know!
Edit: Had a look at the si476x PDF and its controlled via I2C and the audio is delivered via I2S. So other chipsets are probably fairly similar. Start small, find a similar driver that has I2C and I2S, get the I2C talking, get the I2S running - job done!
