drm/amdgpu: expose sclk and mclk via hwmon

Expose sclk (gfx clock) and mclk (memory clock) via
hwmon compatible interface.  hwmon does not actually
formally specify a frequency type attribute, but these
are compatible with the format of the other attributes
exposed via hwmon.  Units are hertz.

freq1_input - GPU gfx/compute clock in hertz
freq2_input - GPU memory clock in hertz (dGPU only)

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