drm/amd/amdgpu: Add a GPU_LOAD entry to sysfs (v3)

This adds what should be a stable interface to read GPU
load from userspace.

(v2): Fix comments and name of file per recommendations.
(v3): Add chapter to amdgpu.rst as well.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Acked-by: Slava Abramov <slava.abramov@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>