Using a high frequency LPDDR3 RAM with a lower RAM frequency supported Processor
In general, if you have a particular speed memory interface, then any memory device that can operate at that speed or faster should work just fine.

This is certainly true in the DDR world (mainly due to it being made to a standard) - I have had to support long term products and the fact is memory devices become obsolete very quickly (simply because the vendors are not interested in making slow devices as the market is not there). The slow devices fall off the product listing.

I simply put in devices capable of higher speeds with no issues at all.

I would check the timing parameters, though (for both memory and processor); the datasheet information is the true arbiter of what will work together.