dev-amdgpu: Add SystemHub for GPU load/store to host

In a dGPU configuration, vector and scalar loads/stores can either be
requests to device memory or host memory depending on if the system bit
is set in the PTE when the request's virtual address is translated. This
object is used to send/receive those requests to the host via DMA.

This object will be used in a later changeset by the compute unit and
fetch units to issue data and instruction loads from the GPU which
translate to physical addresses on the host/cpu memory.

Change-Id: I4537059f90ebc03f3b2e6b8b631b4c452841f83f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/51851
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
4 files changed