dev-amdgpu: Implement MMIO trace reader

Helper class to read Linux kernel MMIO trace from amdgpu modprobes. This
class is used rather than implementing MMIOs in code as it is easier to
update to newer kernel versions this way. It also helps with setting
values for registers which are not documented.

Based on https://gem5-review.googlesource.com/c/amd/gem5/+/23743

Change-Id: Ia9b85c269c98b6ae0d5bcfe89141a4c30ef2f914
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/46160
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/dev/amdgpu/SConscript

@@ -38,5 +38,6 @@ if env['TARGET_ISA'] != 'x86' or not env['BUILD_GPU']:
 SimObject('AMDGPU.py')
 
 Source('amdgpu_device.cc')
+Source('mmio_reader.cc')
 
 DebugFlag('AMDGPUDevice')