Using narrower DDR RAM than controller
Can I buy eg. two 32bit chips and combine them?


I imagine so. The product brief describes ECC as "optional", which indicates to me that it can operate with non-ECC memory:


  The memory system supports the latest DDR4 memories in addition to DDR3 and DDR3L. Optional ECC support is provided as well.


In practice, though, this part appears to be designed to operate with very large quantities of memory. (For instance, it advertises "address space beyond 4 GB".) Using it with the bare minimum of memory seems like it'd be a waste of its capabilities.


  If yes then how?


Contact the vendor for a full datasheet. Keep in mind that Broadcom typically only works with large customers; if you just have a few of these parts from a gray-market source, you are probably out of luck.


  Can I buy just one 32bit and pull down rest of data pins on processor?


You'd have to ask the vendor, but it seems unlikely.