mem: Expose the underlyig DRAM or NVM's memory back door.

mem: Expose the underlyig DRAM or NVM's memory back door.

Use the AbstractMem's new getBackdoor call to implement the
recvAtomicBackdoor call in the memory controller's port.

Change-Id: I10a7d22edb62afc3b77a2d462f297572c04f020d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36978
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>