Actually it really isn't known who all the DRAM suppliers are. There was a recent tear down of the iPhone 5S and it did show Hynix but that does not mean other phones do not have Micron DRAM. It is very very very unlikely that Apple would have one supplier for a commodity part.
You can bank on the fact Apple would not sole-source their mDRAMs. If you open up enough of their phones, you will likely find Samsung, Hynix, Elpida, and even Micron are all represented. Same would be true for their NAND parts. Apple is the #2 buyer of memories out there, they have to have multiple sources.