How does the MMU map multi-channel setups
ASLR has nothing to do with this. ASLR is a software technique to secure the applications where all executables are relocatable in that they keep information in the executable to relocate it to any address.
I would guess that the MMU doesn't see if there are several RAM DIMMs. It is the motherboard's job to make sure that if the memory controller of the CPU sets certain lines of the address bus, they are directed towards the right DIMM for decoding.
In hardware it could look like switches (transistors) activated by higher order bits of the address bus that will redirect all address lines to another DIMM. For example, if you have 2 DIMMs of 8GB each then the first DIMM needs 33 address lines for decoding because 2^33 bytes = 8GB. The second DIMM brings the computer to 16GB so, if you set any line of the address bus above 33, then the motherboard knows it needs to use the second DIMM.