drm/amd/powerplay: read pcie speed/width info (v2)

sysfs interface to read pcie speed&width info on navi1x.

v2: fix warning (trivial)

Signed-off-by: Kenneth Feng <kenneth.feng@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>